libgo: update to Go 1.11

[official-gcc.git] / libgo / go / go / parser / parser.go

// Copyright 2009 The Go Authors. All rights reserved.
// Use of this source code is governed by a BSD-style
// license that can be found in the LICENSE file.

// Package parser implements a parser for Go source files. Input may be
// provided in a variety of forms (see the various Parse* functions); the
// output is an abstract syntax tree (AST) representing the Go source. The
// parser is invoked through one of the Parse* functions.
//
// The parser accepts a larger language than is syntactically permitted by
// the Go spec, for simplicity, and for improved robustness in the presence
// of syntax errors. For instance, in method declarations, the receiver is
// treated like an ordinary parameter list and thus may contain multiple
// entries where the spec permits exactly one. Consequently, the corresponding
// field in the AST (ast.FuncDecl.Recv) field is not restricted to one entry.
//
package parser

import (
        "fmt"
        "go/ast"
        "go/scanner"
        "go/token"
        "strconv"
        "strings"
        "unicode"
)

// The parser structure holds the parser's internal state.
type parser struct {
        file    *token.File
        errors  scanner.ErrorList
        scanner scanner.Scanner

        // Tracing/debugging
        mode   Mode // parsing mode
        trace  bool // == (mode & Trace != 0)
        indent int  // indentation used for tracing output

        // Comments
        comments    []*ast.CommentGroup
        leadComment *ast.CommentGroup // last lead comment
        lineComment *ast.CommentGroup // last line comment

        // Next token
        pos token.Pos   // token position
        tok token.Token // one token look-ahead
        lit string      // token literal

        // Error recovery
        // (used to limit the number of calls to parser.advance
        // w/o making scanning progress - avoids potential endless
        // loops across multiple parser functions during error recovery)
        syncPos token.Pos // last synchronization position
        syncCnt int       // number of parser.advance calls without progress

        // Non-syntactic parser control
        exprLev int  // < 0: in control clause, >= 0: in expression
        inRhs   bool // if set, the parser is parsing a rhs expression

        // Ordinary identifier scopes
        pkgScope   *ast.Scope        // pkgScope.Outer == nil
        topScope   *ast.Scope        // top-most scope; may be pkgScope
        unresolved []*ast.Ident      // unresolved identifiers
        imports    []*ast.ImportSpec // list of imports

        // Label scopes
        // (maintained by open/close LabelScope)
        labelScope  *ast.Scope     // label scope for current function
        targetStack [][]*ast.Ident // stack of unresolved labels
}

func (p *parser) init(fset *token.FileSet, filename string, src []byte, mode Mode) {
        p.file = fset.AddFile(filename, -1, len(src))
        var m scanner.Mode
        if mode&ParseComments != 0 {
                m = scanner.ScanComments
        }
        eh := func(pos token.Position, msg string) { p.errors.Add(pos, msg) }
        p.scanner.Init(p.file, src, eh, m)

        p.mode = mode
        p.trace = mode&Trace != 0 // for convenience (p.trace is used frequently)

        p.next()
}

// ----------------------------------------------------------------------------
// Scoping support

func (p *parser) openScope() {
        p.topScope = ast.NewScope(p.topScope)
}

func (p *parser) closeScope() {
        p.topScope = p.topScope.Outer
}

func (p *parser) openLabelScope() {
        p.labelScope = ast.NewScope(p.labelScope)
        p.targetStack = append(p.targetStack, nil)
}

func (p *parser) closeLabelScope() {
        // resolve labels
        n := len(p.targetStack) - 1
        scope := p.labelScope
        for _, ident := range p.targetStack[n] {
                ident.Obj = scope.Lookup(ident.Name)
                if ident.Obj == nil && p.mode&DeclarationErrors != 0 {
                        p.error(ident.Pos(), fmt.Sprintf("label %s undefined", ident.Name))
                }
        }
        // pop label scope
        p.targetStack = p.targetStack[0:n]
        p.labelScope = p.labelScope.Outer
}

func (p *parser) declare(decl, data interface{}, scope *ast.Scope, kind ast.ObjKind, idents ...*ast.Ident) {
        for _, ident := range idents {
                assert(ident.Obj == nil, "identifier already declared or resolved")
                obj := ast.NewObj(kind, ident.Name)
                // remember the corresponding declaration for redeclaration
                // errors and global variable resolution/typechecking phase
                obj.Decl = decl
                obj.Data = data
                ident.Obj = obj
                if ident.Name != "_" {
                        if alt := scope.Insert(obj); alt != nil && p.mode&DeclarationErrors != 0 {
                                prevDecl := ""
                                if pos := alt.Pos(); pos.IsValid() {
                                        prevDecl = fmt.Sprintf("\n\tprevious declaration at %s", p.file.Position(pos))
                                }
                                p.error(ident.Pos(), fmt.Sprintf("%s redeclared in this block%s", ident.Name, prevDecl))
                        }
                }
        }
}

func (p *parser) shortVarDecl(decl *ast.AssignStmt, list []ast.Expr) {
        // Go spec: A short variable declaration may redeclare variables
        // provided they were originally declared in the same block with
        // the same type, and at least one of the non-blank variables is new.
        n := 0 // number of new variables
        for _, x := range list {
                if ident, isIdent := x.(*ast.Ident); isIdent {
                        assert(ident.Obj == nil, "identifier already declared or resolved")
                        obj := ast.NewObj(ast.Var, ident.Name)
                        // remember corresponding assignment for other tools
                        obj.Decl = decl
                        ident.Obj = obj
                        if ident.Name != "_" {
                                if alt := p.topScope.Insert(obj); alt != nil {
                                        ident.Obj = alt // redeclaration
                                } else {
                                        n++ // new declaration
                                }
                        }
                } else {
                        p.errorExpected(x.Pos(), "identifier on left side of :=")
                }
        }
        if n == 0 && p.mode&DeclarationErrors != 0 {
                p.error(list[0].Pos(), "no new variables on left side of :=")
        }
}

// The unresolved object is a sentinel to mark identifiers that have been added
// to the list of unresolved identifiers. The sentinel is only used for verifying
// internal consistency.
var unresolved = new(ast.Object)

// If x is an identifier, tryResolve attempts to resolve x by looking up
// the object it denotes. If no object is found and collectUnresolved is
// set, x is marked as unresolved and collected in the list of unresolved
// identifiers.
//
func (p *parser) tryResolve(x ast.Expr, collectUnresolved bool) {
        // nothing to do if x is not an identifier or the blank identifier
        ident, _ := x.(*ast.Ident)
        if ident == nil {
                return
        }
        assert(ident.Obj == nil, "identifier already declared or resolved")
        if ident.Name == "_" {
                return
        }
        // try to resolve the identifier
        for s := p.topScope; s != nil; s = s.Outer {
                if obj := s.Lookup(ident.Name); obj != nil {
                        ident.Obj = obj
                        return
                }
        }
        // all local scopes are known, so any unresolved identifier
        // must be found either in the file scope, package scope
        // (perhaps in another file), or universe scope --- collect
        // them so that they can be resolved later
        if collectUnresolved {
                ident.Obj = unresolved
                p.unresolved = append(p.unresolved, ident)
        }
}

func (p *parser) resolve(x ast.Expr) {
        p.tryResolve(x, true)
}

// ----------------------------------------------------------------------------
// Parsing support

func (p *parser) printTrace(a ...interface{}) {
        const dots = ". . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . "
        const n = len(dots)
        pos := p.file.Position(p.pos)
        fmt.Printf("%5d:%3d: ", pos.Line, pos.Column)
        i := 2 * p.indent
        for i > n {
                fmt.Print(dots)
                i -= n
        }
        // i <= n
        fmt.Print(dots[0:i])
        fmt.Println(a...)
}

func trace(p *parser, msg string) *parser {
        p.printTrace(msg, "(")
        p.indent++
        return p
}

// Usage pattern: defer un(trace(p, "..."))
func un(p *parser) {
        p.indent--
        p.printTrace(")")
}

// Advance to the next token.
func (p *parser) next0() {
        // Because of one-token look-ahead, print the previous token
        // when tracing as it provides a more readable output. The
        // very first token (!p.pos.IsValid()) is not initialized
        // (it is token.ILLEGAL), so don't print it .
        if p.trace && p.pos.IsValid() {
                s := p.tok.String()
                switch {
                case p.tok.IsLiteral():
                        p.printTrace(s, p.lit)
                case p.tok.IsOperator(), p.tok.IsKeyword():
                        p.printTrace("\"" + s + "\"")
                default:
                        p.printTrace(s)
                }
        }

        p.pos, p.tok, p.lit = p.scanner.Scan()
}

// Consume a comment and return it and the line on which it ends.
func (p *parser) consumeComment() (comment *ast.Comment, endline int) {
        // /*-style comments may end on a different line than where they start.
        // Scan the comment for '\n' chars and adjust endline accordingly.
        endline = p.file.Line(p.pos)
        if p.lit[1] == '*' {
                // don't use range here - no need to decode Unicode code points
                for i := 0; i < len(p.lit); i++ {
                        if p.lit[i] == '\n' {
                                endline++
                        }
                }
        }

        comment = &ast.Comment{Slash: p.pos, Text: p.lit}
        p.next0()

        return
}

// Consume a group of adjacent comments, add it to the parser's
// comments list, and return it together with the line at which
// the last comment in the group ends. A non-comment token or n
// empty lines terminate a comment group.
//
func (p *parser) consumeCommentGroup(n int) (comments *ast.CommentGroup, endline int) {
        var list []*ast.Comment
        endline = p.file.Line(p.pos)
        for p.tok == token.COMMENT && p.file.Line(p.pos) <= endline+n {
                var comment *ast.Comment
                comment, endline = p.consumeComment()
                list = append(list, comment)
        }

        // add comment group to the comments list
        comments = &ast.CommentGroup{List: list}
        p.comments = append(p.comments, comments)

        return
}

// Advance to the next non-comment token. In the process, collect
// any comment groups encountered, and remember the last lead and
// and line comments.
//
// A lead comment is a comment group that starts and ends in a
// line without any other tokens and that is followed by a non-comment
// token on the line immediately after the comment group.
//
// A line comment is a comment group that follows a non-comment
// token on the same line, and that has no tokens after it on the line
// where it ends.
//
// Lead and line comments may be considered documentation that is
// stored in the AST.
//
func (p *parser) next() {
        p.leadComment = nil
        p.lineComment = nil
        prev := p.pos
        p.next0()

        if p.tok == token.COMMENT {
                var comment *ast.CommentGroup
                var endline int

                if p.file.Line(p.pos) == p.file.Line(prev) {
                        // The comment is on same line as the previous token; it
                        // cannot be a lead comment but may be a line comment.
                        comment, endline = p.consumeCommentGroup(0)
                        if p.file.Line(p.pos) != endline || p.tok == token.EOF {
                                // The next token is on a different line, thus
                                // the last comment group is a line comment.
                                p.lineComment = comment
                        }
                }

                // consume successor comments, if any
                endline = -1
                for p.tok == token.COMMENT {
                        comment, endline = p.consumeCommentGroup(1)
                }

                if endline+1 == p.file.Line(p.pos) {
                        // The next token is following on the line immediately after the
                        // comment group, thus the last comment group is a lead comment.
                        p.leadComment = comment
                }
        }
}

// A bailout panic is raised to indicate early termination.
type bailout struct{}

func (p *parser) error(pos token.Pos, msg string) {
        epos := p.file.Position(pos)

        // If AllErrors is not set, discard errors reported on the same line
        // as the last recorded error and stop parsing if there are more than
        // 10 errors.
        if p.mode&AllErrors == 0 {
                n := len(p.errors)
                if n > 0 && p.errors[n-1].Pos.Line == epos.Line {
                        return // discard - likely a spurious error
                }
                if n > 10 {
                        panic(bailout{})
                }
        }

        p.errors.Add(epos, msg)
}

func (p *parser) errorExpected(pos token.Pos, msg string) {
        msg = "expected " + msg
        if pos == p.pos {
                // the error happened at the current position;
                // make the error message more specific
                switch {
                case p.tok == token.SEMICOLON && p.lit == "\n":
                        msg += ", found newline"
                case p.tok.IsLiteral():
                        // print 123 rather than 'INT', etc.
                        msg += ", found " + p.lit
                default:
                        msg += ", found '" + p.tok.String() + "'"
                }
        }
        p.error(pos, msg)
}

func (p *parser) expect(tok token.Token) token.Pos {
        pos := p.pos
        if p.tok != tok {
                p.errorExpected(pos, "'"+tok.String()+"'")
        }
        p.next() // make progress
        return pos
}

// expectClosing is like expect but provides a better error message
// for the common case of a missing comma before a newline.
//
func (p *parser) expectClosing(tok token.Token, context string) token.Pos {
        if p.tok != tok && p.tok == token.SEMICOLON && p.lit == "\n" {
                p.error(p.pos, "missing ',' before newline in "+context)
                p.next()
        }
        return p.expect(tok)
}

func (p *parser) expectSemi() {
        // semicolon is optional before a closing ')' or '}'
        if p.tok != token.RPAREN && p.tok != token.RBRACE {
                switch p.tok {
                case token.COMMA:
                        // permit a ',' instead of a ';' but complain
                        p.errorExpected(p.pos, "';'")
                        fallthrough
                case token.SEMICOLON:
                        p.next()
                default:
                        p.errorExpected(p.pos, "';'")
                        p.advance(stmtStart)
                }
        }
}

func (p *parser) atComma(context string, follow token.Token) bool {
        if p.tok == token.COMMA {
                return true
        }
        if p.tok != follow {
                msg := "missing ','"
                if p.tok == token.SEMICOLON && p.lit == "\n" {
                        msg += " before newline"
                }
                p.error(p.pos, msg+" in "+context)
                return true // "insert" comma and continue
        }
        return false
}

func assert(cond bool, msg string) {
        if !cond {
                panic("go/parser internal error: " + msg)
        }
}

// advance consumes tokens until the current token p.tok
// is in the 'to' set, or token.EOF. For error recovery.
func (p *parser) advance(to map[token.Token]bool) {
        for ; p.tok != token.EOF; p.next() {
                if to[p.tok] {
                        // Return only if parser made some progress since last
                        // sync or if it has not reached 10 advance calls without
                        // progress. Otherwise consume at least one token to
                        // avoid an endless parser loop (it is possible that
                        // both parseOperand and parseStmt call advance and
                        // correctly do not advance, thus the need for the
                        // invocation limit p.syncCnt).
                        if p.pos == p.syncPos && p.syncCnt < 10 {
                                p.syncCnt++
                                return
                        }
                        if p.pos > p.syncPos {
                                p.syncPos = p.pos
                                p.syncCnt = 0
                                return
                        }
                        // Reaching here indicates a parser bug, likely an
                        // incorrect token list in this function, but it only
                        // leads to skipping of possibly correct code if a
                        // previous error is present, and thus is preferred
                        // over a non-terminating parse.
                }
        }
}

var stmtStart = map[token.Token]bool{
        token.BREAK:       true,
        token.CONST:       true,
        token.CONTINUE:    true,
        token.DEFER:       true,
        token.FALLTHROUGH: true,
        token.FOR:         true,
        token.GO:          true,
        token.GOTO:        true,
        token.IF:          true,
        token.RETURN:      true,
        token.SELECT:      true,
        token.SWITCH:      true,
        token.TYPE:        true,
        token.VAR:         true,
}

var declStart = map[token.Token]bool{
        token.CONST: true,
        token.TYPE:  true,
        token.VAR:   true,
}

var exprEnd = map[token.Token]bool{
        token.COMMA:     true,
        token.COLON:     true,
        token.SEMICOLON: true,
        token.RPAREN:    true,
        token.RBRACK:    true,
        token.RBRACE:    true,
}

// safePos returns a valid file position for a given position: If pos
// is valid to begin with, safePos returns pos. If pos is out-of-range,
// safePos returns the EOF position.
//
// This is hack to work around "artificial" end positions in the AST which
// are computed by adding 1 to (presumably valid) token positions. If the
// token positions are invalid due to parse errors, the resulting end position
// may be past the file's EOF position, which would lead to panics if used
// later on.
//
func (p *parser) safePos(pos token.Pos) (res token.Pos) {
        defer func() {
                if recover() != nil {
                        res = token.Pos(p.file.Base() + p.file.Size()) // EOF position
                }
        }()
        _ = p.file.Offset(pos) // trigger a panic if position is out-of-range
        return pos
}

// ----------------------------------------------------------------------------
// Identifiers

func (p *parser) parseIdent() *ast.Ident {
        pos := p.pos
        name := "_"
        if p.tok == token.IDENT {
                name = p.lit
                p.next()
        } else {
                p.expect(token.IDENT) // use expect() error handling
        }
        return &ast.Ident{NamePos: pos, Name: name}
}

func (p *parser) parseIdentList() (list []*ast.Ident) {
        if p.trace {
                defer un(trace(p, "IdentList"))
        }

        list = append(list, p.parseIdent())
        for p.tok == token.COMMA {
                p.next()
                list = append(list, p.parseIdent())
        }

        return
}

// ----------------------------------------------------------------------------
// Common productions

// If lhs is set, result list elements which are identifiers are not resolved.
func (p *parser) parseExprList(lhs bool) (list []ast.Expr) {
        if p.trace {
                defer un(trace(p, "ExpressionList"))
        }

        list = append(list, p.checkExpr(p.parseExpr(lhs)))
        for p.tok == token.COMMA {
                p.next()
                list = append(list, p.checkExpr(p.parseExpr(lhs)))
        }

        return
}

func (p *parser) parseLhsList() []ast.Expr {
        old := p.inRhs
        p.inRhs = false
        list := p.parseExprList(true)
        switch p.tok {
        case token.DEFINE:
                // lhs of a short variable declaration
                // but doesn't enter scope until later:
                // caller must call p.shortVarDecl(p.makeIdentList(list))
                // at appropriate time.
        case token.COLON:
                // lhs of a label declaration or a communication clause of a select
                // statement (parseLhsList is not called when parsing the case clause
                // of a switch statement):
                // - labels are declared by the caller of parseLhsList
                // - for communication clauses, if there is a stand-alone identifier
                //   followed by a colon, we have a syntax error; there is no need
                //   to resolve the identifier in that case
        default:
                // identifiers must be declared elsewhere
                for _, x := range list {
                        p.resolve(x)
                }
        }
        p.inRhs = old
        return list
}

func (p *parser) parseRhsList() []ast.Expr {
        old := p.inRhs
        p.inRhs = true
        list := p.parseExprList(false)
        p.inRhs = old
        return list
}

// ----------------------------------------------------------------------------
// Types

func (p *parser) parseType() ast.Expr {
        if p.trace {
                defer un(trace(p, "Type"))
        }

        typ := p.tryType()

        if typ == nil {
                pos := p.pos
                p.errorExpected(pos, "type")
                p.advance(exprEnd)
                return &ast.BadExpr{From: pos, To: p.pos}
        }

        return typ
}

// If the result is an identifier, it is not resolved.
func (p *parser) parseTypeName() ast.Expr {
        if p.trace {
                defer un(trace(p, "TypeName"))
        }

        ident := p.parseIdent()
        // don't resolve ident yet - it may be a parameter or field name

        if p.tok == token.PERIOD {
                // ident is a package name
                p.next()
                p.resolve(ident)
                sel := p.parseIdent()
                return &ast.SelectorExpr{X: ident, Sel: sel}
        }

        return ident
}

func (p *parser) parseArrayType() ast.Expr {
        if p.trace {
                defer un(trace(p, "ArrayType"))
        }

        lbrack := p.expect(token.LBRACK)
        p.exprLev++
        var len ast.Expr
        // always permit ellipsis for more fault-tolerant parsing
        if p.tok == token.ELLIPSIS {
                len = &ast.Ellipsis{Ellipsis: p.pos}
                p.next()
        } else if p.tok != token.RBRACK {
                len = p.parseRhs()
        }
        p.exprLev--
        p.expect(token.RBRACK)
        elt := p.parseType()

        return &ast.ArrayType{Lbrack: lbrack, Len: len, Elt: elt}
}

func (p *parser) makeIdentList(list []ast.Expr) []*ast.Ident {
        idents := make([]*ast.Ident, len(list))
        for i, x := range list {
                ident, isIdent := x.(*ast.Ident)
                if !isIdent {
                        if _, isBad := x.(*ast.BadExpr); !isBad {
                                // only report error if it's a new one
                                p.errorExpected(x.Pos(), "identifier")
                        }
                        ident = &ast.Ident{NamePos: x.Pos(), Name: "_"}
                }
                idents[i] = ident
        }
        return idents
}

func (p *parser) parseFieldDecl(scope *ast.Scope) *ast.Field {
        if p.trace {
                defer un(trace(p, "FieldDecl"))
        }

        doc := p.leadComment

        // 1st FieldDecl
        // A type name used as an anonymous field looks like a field identifier.
        var list []ast.Expr
        for {
                list = append(list, p.parseVarType(false))
                if p.tok != token.COMMA {
                        break
                }
                p.next()
        }

        typ := p.tryVarType(false)

        // analyze case
        var idents []*ast.Ident
        if typ != nil {
                // IdentifierList Type
                idents = p.makeIdentList(list)
        } else {
                // ["*"] TypeName (AnonymousField)
                typ = list[0] // we always have at least one element
                if n := len(list); n > 1 {
                        p.errorExpected(p.pos, "type")
                        typ = &ast.BadExpr{From: p.pos, To: p.pos}
                } else if !isTypeName(deref(typ)) {
                        p.errorExpected(typ.Pos(), "anonymous field")
                        typ = &ast.BadExpr{From: typ.Pos(), To: p.safePos(typ.End())}
                }
        }

        // Tag
        var tag *ast.BasicLit
        if p.tok == token.STRING {
                tag = &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
                p.next()
        }

        p.expectSemi() // call before accessing p.linecomment

        field := &ast.Field{Doc: doc, Names: idents, Type: typ, Tag: tag, Comment: p.lineComment}
        p.declare(field, nil, scope, ast.Var, idents...)
        p.resolve(typ)

        return field
}

func (p *parser) parseStructType() *ast.StructType {
        if p.trace {
                defer un(trace(p, "StructType"))
        }

        pos := p.expect(token.STRUCT)
        lbrace := p.expect(token.LBRACE)
        scope := ast.NewScope(nil) // struct scope
        var list []*ast.Field
        for p.tok == token.IDENT || p.tok == token.MUL || p.tok == token.LPAREN {
                // a field declaration cannot start with a '(' but we accept
                // it here for more robust parsing and better error messages
                // (parseFieldDecl will check and complain if necessary)
                list = append(list, p.parseFieldDecl(scope))
        }
        rbrace := p.expect(token.RBRACE)

        return &ast.StructType{
                Struct: pos,
                Fields: &ast.FieldList{
                        Opening: lbrace,
                        List:    list,
                        Closing: rbrace,
                },
        }
}

func (p *parser) parsePointerType() *ast.StarExpr {
        if p.trace {
                defer un(trace(p, "PointerType"))
        }

        star := p.expect(token.MUL)
        base := p.parseType()

        return &ast.StarExpr{Star: star, X: base}
}

// If the result is an identifier, it is not resolved.
func (p *parser) tryVarType(isParam bool) ast.Expr {
        if isParam && p.tok == token.ELLIPSIS {
                pos := p.pos
                p.next()
                typ := p.tryIdentOrType() // don't use parseType so we can provide better error message
                if typ != nil {
                        p.resolve(typ)
                } else {
                        p.error(pos, "'...' parameter is missing type")
                        typ = &ast.BadExpr{From: pos, To: p.pos}
                }
                return &ast.Ellipsis{Ellipsis: pos, Elt: typ}
        }
        return p.tryIdentOrType()
}

// If the result is an identifier, it is not resolved.
func (p *parser) parseVarType(isParam bool) ast.Expr {
        typ := p.tryVarType(isParam)
        if typ == nil {
                pos := p.pos
                p.errorExpected(pos, "type")
                p.next() // make progress
                typ = &ast.BadExpr{From: pos, To: p.pos}
        }
        return typ
}

func (p *parser) parseParameterList(scope *ast.Scope, ellipsisOk bool) (params []*ast.Field) {
        if p.trace {
                defer un(trace(p, "ParameterList"))
        }

        // 1st ParameterDecl
        // A list of identifiers looks like a list of type names.
        var list []ast.Expr
        for {
                list = append(list, p.parseVarType(ellipsisOk))
                if p.tok != token.COMMA {
                        break
                }
                p.next()
                if p.tok == token.RPAREN {
                        break
                }
        }

        // analyze case
        if typ := p.tryVarType(ellipsisOk); typ != nil {
                // IdentifierList Type
                idents := p.makeIdentList(list)
                field := &ast.Field{Names: idents, Type: typ}
                params = append(params, field)
                // Go spec: The scope of an identifier denoting a function
                // parameter or result variable is the function body.
                p.declare(field, nil, scope, ast.Var, idents...)
                p.resolve(typ)
                if !p.atComma("parameter list", token.RPAREN) {
                        return
                }
                p.next()
                for p.tok != token.RPAREN && p.tok != token.EOF {
                        idents := p.parseIdentList()
                        typ := p.parseVarType(ellipsisOk)
                        field := &ast.Field{Names: idents, Type: typ}
                        params = append(params, field)
                        // Go spec: The scope of an identifier denoting a function
                        // parameter or result variable is the function body.
                        p.declare(field, nil, scope, ast.Var, idents...)
                        p.resolve(typ)
                        if !p.atComma("parameter list", token.RPAREN) {
                                break
                        }
                        p.next()
                }
                return
        }

        // Type { "," Type } (anonymous parameters)
        params = make([]*ast.Field, len(list))
        for i, typ := range list {
                p.resolve(typ)
                params[i] = &ast.Field{Type: typ}
        }
        return
}

func (p *parser) parseParameters(scope *ast.Scope, ellipsisOk bool) *ast.FieldList {
        if p.trace {
                defer un(trace(p, "Parameters"))
        }

        var params []*ast.Field
        lparen := p.expect(token.LPAREN)
        if p.tok != token.RPAREN {
                params = p.parseParameterList(scope, ellipsisOk)
        }
        rparen := p.expect(token.RPAREN)

        return &ast.FieldList{Opening: lparen, List: params, Closing: rparen}
}

func (p *parser) parseResult(scope *ast.Scope) *ast.FieldList {
        if p.trace {
                defer un(trace(p, "Result"))
        }

        if p.tok == token.LPAREN {
                return p.parseParameters(scope, false)
        }

        typ := p.tryType()
        if typ != nil {
                list := make([]*ast.Field, 1)
                list[0] = &ast.Field{Type: typ}
                return &ast.FieldList{List: list}
        }

        return nil
}

func (p *parser) parseSignature(scope *ast.Scope) (params, results *ast.FieldList) {
        if p.trace {
                defer un(trace(p, "Signature"))
        }

        params = p.parseParameters(scope, true)
        results = p.parseResult(scope)

        return
}

func (p *parser) parseFuncType() (*ast.FuncType, *ast.Scope) {
        if p.trace {
                defer un(trace(p, "FuncType"))
        }

        pos := p.expect(token.FUNC)
        scope := ast.NewScope(p.topScope) // function scope
        params, results := p.parseSignature(scope)

        return &ast.FuncType{Func: pos, Params: params, Results: results}, scope
}

func (p *parser) parseMethodSpec(scope *ast.Scope) *ast.Field {
        if p.trace {
                defer un(trace(p, "MethodSpec"))
        }

        doc := p.leadComment
        var idents []*ast.Ident
        var typ ast.Expr
        x := p.parseTypeName()
        if ident, isIdent := x.(*ast.Ident); isIdent && p.tok == token.LPAREN {
                // method
                idents = []*ast.Ident{ident}
                scope := ast.NewScope(nil) // method scope
                params, results := p.parseSignature(scope)
                typ = &ast.FuncType{Func: token.NoPos, Params: params, Results: results}
        } else {
                // embedded interface
                typ = x
                p.resolve(typ)
        }
        p.expectSemi() // call before accessing p.linecomment

        spec := &ast.Field{Doc: doc, Names: idents, Type: typ, Comment: p.lineComment}
        p.declare(spec, nil, scope, ast.Fun, idents...)

        return spec
}

func (p *parser) parseInterfaceType() *ast.InterfaceType {
        if p.trace {
                defer un(trace(p, "InterfaceType"))
        }

        pos := p.expect(token.INTERFACE)
        lbrace := p.expect(token.LBRACE)
        scope := ast.NewScope(nil) // interface scope
        var list []*ast.Field
        for p.tok == token.IDENT {
                list = append(list, p.parseMethodSpec(scope))
        }
        rbrace := p.expect(token.RBRACE)

        return &ast.InterfaceType{
                Interface: pos,
                Methods: &ast.FieldList{
                        Opening: lbrace,
                        List:    list,
                        Closing: rbrace,
                },
        }
}

func (p *parser) parseMapType() *ast.MapType {
        if p.trace {
                defer un(trace(p, "MapType"))
        }

        pos := p.expect(token.MAP)
        p.expect(token.LBRACK)
        key := p.parseType()
        p.expect(token.RBRACK)
        value := p.parseType()

        return &ast.MapType{Map: pos, Key: key, Value: value}
}

func (p *parser) parseChanType() *ast.ChanType {
        if p.trace {
                defer un(trace(p, "ChanType"))
        }

        pos := p.pos
        dir := ast.SEND | ast.RECV
        var arrow token.Pos
        if p.tok == token.CHAN {
                p.next()
                if p.tok == token.ARROW {
                        arrow = p.pos
                        p.next()
                        dir = ast.SEND
                }
        } else {
                arrow = p.expect(token.ARROW)
                p.expect(token.CHAN)
                dir = ast.RECV
        }
        value := p.parseType()

        return &ast.ChanType{Begin: pos, Arrow: arrow, Dir: dir, Value: value}
}

// If the result is an identifier, it is not resolved.
func (p *parser) tryIdentOrType() ast.Expr {
        switch p.tok {
        case token.IDENT:
                return p.parseTypeName()
        case token.LBRACK:
                return p.parseArrayType()
        case token.STRUCT:
                return p.parseStructType()
        case token.MUL:
                return p.parsePointerType()
        case token.FUNC:
                typ, _ := p.parseFuncType()
                return typ
        case token.INTERFACE:
                return p.parseInterfaceType()
        case token.MAP:
                return p.parseMapType()
        case token.CHAN, token.ARROW:
                return p.parseChanType()
        case token.LPAREN:
                lparen := p.pos
                p.next()
                typ := p.parseType()
                rparen := p.expect(token.RPAREN)
                return &ast.ParenExpr{Lparen: lparen, X: typ, Rparen: rparen}
        }

        // no type found
        return nil
}

func (p *parser) tryType() ast.Expr {
        typ := p.tryIdentOrType()
        if typ != nil {
                p.resolve(typ)
        }
        return typ
}

// ----------------------------------------------------------------------------
// Blocks

func (p *parser) parseStmtList() (list []ast.Stmt) {
        if p.trace {
                defer un(trace(p, "StatementList"))
        }

        for p.tok != token.CASE && p.tok != token.DEFAULT && p.tok != token.RBRACE && p.tok != token.EOF {
                list = append(list, p.parseStmt())
        }

        return
}

func (p *parser) parseBody(scope *ast.Scope) *ast.BlockStmt {
        if p.trace {
                defer un(trace(p, "Body"))
        }

        lbrace := p.expect(token.LBRACE)
        p.topScope = scope // open function scope
        p.openLabelScope()
        list := p.parseStmtList()
        p.closeLabelScope()
        p.closeScope()
        rbrace := p.expect(token.RBRACE)

        return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
}

func (p *parser) parseBlockStmt() *ast.BlockStmt {
        if p.trace {
                defer un(trace(p, "BlockStmt"))
        }

        lbrace := p.expect(token.LBRACE)
        p.openScope()
        list := p.parseStmtList()
        p.closeScope()
        rbrace := p.expect(token.RBRACE)

        return &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}
}

// ----------------------------------------------------------------------------
// Expressions

func (p *parser) parseFuncTypeOrLit() ast.Expr {
        if p.trace {
                defer un(trace(p, "FuncTypeOrLit"))
        }

        typ, scope := p.parseFuncType()
        if p.tok != token.LBRACE {
                // function type only
                return typ
        }

        p.exprLev++
        body := p.parseBody(scope)
        p.exprLev--

        return &ast.FuncLit{Type: typ, Body: body}
}

// parseOperand may return an expression or a raw type (incl. array
// types of the form [...]T. Callers must verify the result.
// If lhs is set and the result is an identifier, it is not resolved.
//
func (p *parser) parseOperand(lhs bool) ast.Expr {
        if p.trace {
                defer un(trace(p, "Operand"))
        }

        switch p.tok {
        case token.IDENT:
                x := p.parseIdent()
                if !lhs {
                        p.resolve(x)
                }
                return x

        case token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING:
                x := &ast.BasicLit{ValuePos: p.pos, Kind: p.tok, Value: p.lit}
                p.next()
                return x

        case token.LPAREN:
                lparen := p.pos
                p.next()
                p.exprLev++
                x := p.parseRhsOrType() // types may be parenthesized: (some type)
                p.exprLev--
                rparen := p.expect(token.RPAREN)
                return &ast.ParenExpr{Lparen: lparen, X: x, Rparen: rparen}

        case token.FUNC:
                return p.parseFuncTypeOrLit()
        }

        if typ := p.tryIdentOrType(); typ != nil {
                // could be type for composite literal or conversion
                _, isIdent := typ.(*ast.Ident)
                assert(!isIdent, "type cannot be identifier")
                return typ
        }

        // we have an error
        pos := p.pos
        p.errorExpected(pos, "operand")
        p.advance(stmtStart)
        return &ast.BadExpr{From: pos, To: p.pos}
}

func (p *parser) parseSelector(x ast.Expr) ast.Expr {
        if p.trace {
                defer un(trace(p, "Selector"))
        }

        sel := p.parseIdent()

        return &ast.SelectorExpr{X: x, Sel: sel}
}

func (p *parser) parseTypeAssertion(x ast.Expr) ast.Expr {
        if p.trace {
                defer un(trace(p, "TypeAssertion"))
        }

        lparen := p.expect(token.LPAREN)
        var typ ast.Expr
        if p.tok == token.TYPE {
                // type switch: typ == nil
                p.next()
        } else {
                typ = p.parseType()
        }
        rparen := p.expect(token.RPAREN)

        return &ast.TypeAssertExpr{X: x, Type: typ, Lparen: lparen, Rparen: rparen}
}

func (p *parser) parseIndexOrSlice(x ast.Expr) ast.Expr {
        if p.trace {
                defer un(trace(p, "IndexOrSlice"))
        }

        const N = 3 // change the 3 to 2 to disable 3-index slices
        lbrack := p.expect(token.LBRACK)
        p.exprLev++
        var index [N]ast.Expr
        var colons [N - 1]token.Pos
        if p.tok != token.COLON {
                index[0] = p.parseRhs()
        }
        ncolons := 0
        for p.tok == token.COLON && ncolons < len(colons) {
                colons[ncolons] = p.pos
                ncolons++
                p.next()
                if p.tok != token.COLON && p.tok != token.RBRACK && p.tok != token.EOF {
                        index[ncolons] = p.parseRhs()
                }
        }
        p.exprLev--
        rbrack := p.expect(token.RBRACK)

        if ncolons > 0 {
                // slice expression
                slice3 := false
                if ncolons == 2 {
                        slice3 = true
                        // Check presence of 2nd and 3rd index here rather than during type-checking
                        // to prevent erroneous programs from passing through gofmt (was issue 7305).
                        if index[1] == nil {
                                p.error(colons[0], "2nd index required in 3-index slice")
                                index[1] = &ast.BadExpr{From: colons[0] + 1, To: colons[1]}
                        }
                        if index[2] == nil {
                                p.error(colons[1], "3rd index required in 3-index slice")
                                index[2] = &ast.BadExpr{From: colons[1] + 1, To: rbrack}
                        }
                }
                return &ast.SliceExpr{X: x, Lbrack: lbrack, Low: index[0], High: index[1], Max: index[2], Slice3: slice3, Rbrack: rbrack}
        }

        return &ast.IndexExpr{X: x, Lbrack: lbrack, Index: index[0], Rbrack: rbrack}
}

func (p *parser) parseCallOrConversion(fun ast.Expr) *ast.CallExpr {
        if p.trace {
                defer un(trace(p, "CallOrConversion"))
        }

        lparen := p.expect(token.LPAREN)
        p.exprLev++
        var list []ast.Expr
        var ellipsis token.Pos
        for p.tok != token.RPAREN && p.tok != token.EOF && !ellipsis.IsValid() {
                list = append(list, p.parseRhsOrType()) // builtins may expect a type: make(some type, ...)
                if p.tok == token.ELLIPSIS {
                        ellipsis = p.pos
                        p.next()
                }
                if !p.atComma("argument list", token.RPAREN) {
                        break
                }
                p.next()
        }
        p.exprLev--
        rparen := p.expectClosing(token.RPAREN, "argument list")

        return &ast.CallExpr{Fun: fun, Lparen: lparen, Args: list, Ellipsis: ellipsis, Rparen: rparen}
}

func (p *parser) parseValue(keyOk bool) ast.Expr {
        if p.trace {
                defer un(trace(p, "Element"))
        }

        if p.tok == token.LBRACE {
                return p.parseLiteralValue(nil)
        }

        // Because the parser doesn't know the composite literal type, it cannot
        // know if a key that's an identifier is a struct field name or a name
        // denoting a value. The former is not resolved by the parser or the
        // resolver.
        //
        // Instead, _try_ to resolve such a key if possible. If it resolves,
        // it a) has correctly resolved, or b) incorrectly resolved because
        // the key is a struct field with a name matching another identifier.
        // In the former case we are done, and in the latter case we don't
        // care because the type checker will do a separate field lookup.
        //
        // If the key does not resolve, it a) must be defined at the top
        // level in another file of the same package, the universe scope, or be
        // undeclared; or b) it is a struct field. In the former case, the type
        // checker can do a top-level lookup, and in the latter case it will do
        // a separate field lookup.
        x := p.checkExpr(p.parseExpr(keyOk))
        if keyOk {
                if p.tok == token.COLON {
                        // Try to resolve the key but don't collect it
                        // as unresolved identifier if it fails so that
                        // we don't get (possibly false) errors about
                        // undeclared names.
                        p.tryResolve(x, false)
                } else {
                        // not a key
                        p.resolve(x)
                }
        }

        return x
}

func (p *parser) parseElement() ast.Expr {
        if p.trace {
                defer un(trace(p, "Element"))
        }

        x := p.parseValue(true)
        if p.tok == token.COLON {
                colon := p.pos
                p.next()
                x = &ast.KeyValueExpr{Key: x, Colon: colon, Value: p.parseValue(false)}
        }

        return x
}

func (p *parser) parseElementList() (list []ast.Expr) {
        if p.trace {
                defer un(trace(p, "ElementList"))
        }

        for p.tok != token.RBRACE && p.tok != token.EOF {
                list = append(list, p.parseElement())
                if !p.atComma("composite literal", token.RBRACE) {
                        break
                }
                p.next()
        }

        return
}

func (p *parser) parseLiteralValue(typ ast.Expr) ast.Expr {
        if p.trace {
                defer un(trace(p, "LiteralValue"))
        }

        lbrace := p.expect(token.LBRACE)
        var elts []ast.Expr
        p.exprLev++
        if p.tok != token.RBRACE {
                elts = p.parseElementList()
        }
        p.exprLev--
        rbrace := p.expectClosing(token.RBRACE, "composite literal")
        return &ast.CompositeLit{Type: typ, Lbrace: lbrace, Elts: elts, Rbrace: rbrace}
}

// checkExpr checks that x is an expression (and not a type).
func (p *parser) checkExpr(x ast.Expr) ast.Expr {
        switch unparen(x).(type) {
        case *ast.BadExpr:
        case *ast.Ident:
        case *ast.BasicLit:
        case *ast.FuncLit:
        case *ast.CompositeLit:
        case *ast.ParenExpr:
                panic("unreachable")
        case *ast.SelectorExpr:
        case *ast.IndexExpr:
        case *ast.SliceExpr:
        case *ast.TypeAssertExpr:
                // If t.Type == nil we have a type assertion of the form
                // y.(type), which is only allowed in type switch expressions.
                // It's hard to exclude those but for the case where we are in
                // a type switch. Instead be lenient and test this in the type
                // checker.
        case *ast.CallExpr:
        case *ast.StarExpr:
        case *ast.UnaryExpr:
        case *ast.BinaryExpr:
        default:
                // all other nodes are not proper expressions
                p.errorExpected(x.Pos(), "expression")
                x = &ast.BadExpr{From: x.Pos(), To: p.safePos(x.End())}
        }
        return x
}

// isTypeName reports whether x is a (qualified) TypeName.
func isTypeName(x ast.Expr) bool {
        switch t := x.(type) {
        case *ast.BadExpr:
        case *ast.Ident:
        case *ast.SelectorExpr:
                _, isIdent := t.X.(*ast.Ident)
                return isIdent
        default:
                return false // all other nodes are not type names
        }
        return true
}

// isLiteralType reports whether x is a legal composite literal type.
func isLiteralType(x ast.Expr) bool {
        switch t := x.(type) {
        case *ast.BadExpr:
        case *ast.Ident:
        case *ast.SelectorExpr:
                _, isIdent := t.X.(*ast.Ident)
                return isIdent
        case *ast.ArrayType:
        case *ast.StructType:
        case *ast.MapType:
        default:
                return false // all other nodes are not legal composite literal types
        }
        return true
}

// If x is of the form *T, deref returns T, otherwise it returns x.
func deref(x ast.Expr) ast.Expr {
        if p, isPtr := x.(*ast.StarExpr); isPtr {
                x = p.X
        }
        return x
}

// If x is of the form (T), unparen returns unparen(T), otherwise it returns x.
func unparen(x ast.Expr) ast.Expr {
        if p, isParen := x.(*ast.ParenExpr); isParen {
                x = unparen(p.X)
        }
        return x
}

// checkExprOrType checks that x is an expression or a type
// (and not a raw type such as [...]T).
//
func (p *parser) checkExprOrType(x ast.Expr) ast.Expr {
        switch t := unparen(x).(type) {
        case *ast.ParenExpr:
                panic("unreachable")
        case *ast.UnaryExpr:
        case *ast.ArrayType:
                if len, isEllipsis := t.Len.(*ast.Ellipsis); isEllipsis {
                        p.error(len.Pos(), "expected array length, found '...'")
                        x = &ast.BadExpr{From: x.Pos(), To: p.safePos(x.End())}
                }
        }

        // all other nodes are expressions or types
        return x
}

// If lhs is set and the result is an identifier, it is not resolved.
func (p *parser) parsePrimaryExpr(lhs bool) ast.Expr {
        if p.trace {
                defer un(trace(p, "PrimaryExpr"))
        }

        x := p.parseOperand(lhs)
L:
        for {
                switch p.tok {
                case token.PERIOD:
                        p.next()
                        if lhs {
                                p.resolve(x)
                        }
                        switch p.tok {
                        case token.IDENT:
                                x = p.parseSelector(p.checkExprOrType(x))
                        case token.LPAREN:
                                x = p.parseTypeAssertion(p.checkExpr(x))
                        default:
                                pos := p.pos
                                p.errorExpected(pos, "selector or type assertion")
                                p.next() // make progress
                                sel := &ast.Ident{NamePos: pos, Name: "_"}
                                x = &ast.SelectorExpr{X: x, Sel: sel}
                        }
                case token.LBRACK:
                        if lhs {
                                p.resolve(x)
                        }
                        x = p.parseIndexOrSlice(p.checkExpr(x))
                case token.LPAREN:
                        if lhs {
                                p.resolve(x)
                        }
                        x = p.parseCallOrConversion(p.checkExprOrType(x))
                case token.LBRACE:
                        if isLiteralType(x) && (p.exprLev >= 0 || !isTypeName(x)) {
                                if lhs {
                                        p.resolve(x)
                                }
                                x = p.parseLiteralValue(x)
                        } else {
                                break L
                        }
                default:
                        break L
                }
                lhs = false // no need to try to resolve again
        }

        return x
}

// If lhs is set and the result is an identifier, it is not resolved.
func (p *parser) parseUnaryExpr(lhs bool) ast.Expr {
        if p.trace {
                defer un(trace(p, "UnaryExpr"))
        }

        switch p.tok {
        case token.ADD, token.SUB, token.NOT, token.XOR, token.AND:
                pos, op := p.pos, p.tok
                p.next()
                x := p.parseUnaryExpr(false)
                return &ast.UnaryExpr{OpPos: pos, Op: op, X: p.checkExpr(x)}

        case token.ARROW:
                // channel type or receive expression
                arrow := p.pos
                p.next()

                // If the next token is token.CHAN we still don't know if it
                // is a channel type or a receive operation - we only know
                // once we have found the end of the unary expression. There
                // are two cases:
                //
                //   <- type  => (<-type) must be channel type
                //   <- expr  => <-(expr) is a receive from an expression
                //
                // In the first case, the arrow must be re-associated with
                // the channel type parsed already:
                //
                //   <- (chan type)    =>  (<-chan type)
                //   <- (chan<- type)  =>  (<-chan (<-type))

                x := p.parseUnaryExpr(false)

                // determine which case we have
                if typ, ok := x.(*ast.ChanType); ok {
                        // (<-type)

                        // re-associate position info and <-
                        dir := ast.SEND
                        for ok && dir == ast.SEND {
                                if typ.Dir == ast.RECV {
                                        // error: (<-type) is (<-(<-chan T))
                                        p.errorExpected(typ.Arrow, "'chan'")
                                }
                                arrow, typ.Begin, typ.Arrow = typ.Arrow, arrow, arrow
                                dir, typ.Dir = typ.Dir, ast.RECV
                                typ, ok = typ.Value.(*ast.ChanType)
                        }
                        if dir == ast.SEND {
                                p.errorExpected(arrow, "channel type")
                        }

                        return x
                }

                // <-(expr)
                return &ast.UnaryExpr{OpPos: arrow, Op: token.ARROW, X: p.checkExpr(x)}

        case token.MUL:
                // pointer type or unary "*" expression
                pos := p.pos
                p.next()
                x := p.parseUnaryExpr(false)
                return &ast.StarExpr{Star: pos, X: p.checkExprOrType(x)}
        }

        return p.parsePrimaryExpr(lhs)
}

func (p *parser) tokPrec() (token.Token, int) {
        tok := p.tok
        if p.inRhs && tok == token.ASSIGN {
                tok = token.EQL
        }
        return tok, tok.Precedence()
}

// If lhs is set and the result is an identifier, it is not resolved.
func (p *parser) parseBinaryExpr(lhs bool, prec1 int) ast.Expr {
        if p.trace {
                defer un(trace(p, "BinaryExpr"))
        }

        x := p.parseUnaryExpr(lhs)
        for {
                op, oprec := p.tokPrec()
                if oprec < prec1 {
                        return x
                }
                pos := p.expect(op)
                if lhs {
                        p.resolve(x)
                        lhs = false
                }
                y := p.parseBinaryExpr(false, oprec+1)
                x = &ast.BinaryExpr{X: p.checkExpr(x), OpPos: pos, Op: op, Y: p.checkExpr(y)}
        }
}

// If lhs is set and the result is an identifier, it is not resolved.
// The result may be a type or even a raw type ([...]int). Callers must
// check the result (using checkExpr or checkExprOrType), depending on
// context.
func (p *parser) parseExpr(lhs bool) ast.Expr {
        if p.trace {
                defer un(trace(p, "Expression"))
        }

        return p.parseBinaryExpr(lhs, token.LowestPrec+1)
}

func (p *parser) parseRhs() ast.Expr {
        old := p.inRhs
        p.inRhs = true
        x := p.checkExpr(p.parseExpr(false))
        p.inRhs = old
        return x
}

func (p *parser) parseRhsOrType() ast.Expr {
        old := p.inRhs
        p.inRhs = true
        x := p.checkExprOrType(p.parseExpr(false))
        p.inRhs = old
        return x
}

// ----------------------------------------------------------------------------
// Statements

// Parsing modes for parseSimpleStmt.
const (
        basic = iota
        labelOk
        rangeOk
)

// parseSimpleStmt returns true as 2nd result if it parsed the assignment
// of a range clause (with mode == rangeOk). The returned statement is an
// assignment with a right-hand side that is a single unary expression of
// the form "range x". No guarantees are given for the left-hand side.
func (p *parser) parseSimpleStmt(mode int) (ast.Stmt, bool) {
        if p.trace {
                defer un(trace(p, "SimpleStmt"))
        }

        x := p.parseLhsList()

        switch p.tok {
        case
                token.DEFINE, token.ASSIGN, token.ADD_ASSIGN,
                token.SUB_ASSIGN, token.MUL_ASSIGN, token.QUO_ASSIGN,
                token.REM_ASSIGN, token.AND_ASSIGN, token.OR_ASSIGN,
                token.XOR_ASSIGN, token.SHL_ASSIGN, token.SHR_ASSIGN, token.AND_NOT_ASSIGN:
                // assignment statement, possibly part of a range clause
                pos, tok := p.pos, p.tok
                p.next()
                var y []ast.Expr
                isRange := false
                if mode == rangeOk && p.tok == token.RANGE && (tok == token.DEFINE || tok == token.ASSIGN) {
                        pos := p.pos
                        p.next()
                        y = []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
                        isRange = true
                } else {
                        y = p.parseRhsList()
                }
                as := &ast.AssignStmt{Lhs: x, TokPos: pos, Tok: tok, Rhs: y}
                if tok == token.DEFINE {
                        p.shortVarDecl(as, x)
                }
                return as, isRange
        }

        if len(x) > 1 {
                p.errorExpected(x[0].Pos(), "1 expression")
                // continue with first expression
        }

        switch p.tok {
        case token.COLON:
                // labeled statement
                colon := p.pos
                p.next()
                if label, isIdent := x[0].(*ast.Ident); mode == labelOk && isIdent {
                        // Go spec: The scope of a label is the body of the function
                        // in which it is declared and excludes the body of any nested
                        // function.
                        stmt := &ast.LabeledStmt{Label: label, Colon: colon, Stmt: p.parseStmt()}
                        p.declare(stmt, nil, p.labelScope, ast.Lbl, label)
                        return stmt, false
                }
                // The label declaration typically starts at x[0].Pos(), but the label
                // declaration may be erroneous due to a token after that position (and
                // before the ':'). If SpuriousErrors is not set, the (only) error
                // reported for the line is the illegal label error instead of the token
                // before the ':' that caused the problem. Thus, use the (latest) colon
                // position for error reporting.
                p.error(colon, "illegal label declaration")
                return &ast.BadStmt{From: x[0].Pos(), To: colon + 1}, false

        case token.ARROW:
                // send statement
                arrow := p.pos
                p.next()
                y := p.parseRhs()
                return &ast.SendStmt{Chan: x[0], Arrow: arrow, Value: y}, false

        case token.INC, token.DEC:
                // increment or decrement
                s := &ast.IncDecStmt{X: x[0], TokPos: p.pos, Tok: p.tok}
                p.next()
                return s, false
        }

        // expression
        return &ast.ExprStmt{X: x[0]}, false
}

func (p *parser) parseCallExpr(callType string) *ast.CallExpr {
        x := p.parseRhsOrType() // could be a conversion: (some type)(x)
        if call, isCall := x.(*ast.CallExpr); isCall {
                return call
        }
        if _, isBad := x.(*ast.BadExpr); !isBad {
                // only report error if it's a new one
                p.error(p.safePos(x.End()), fmt.Sprintf("function must be invoked in %s statement", callType))
        }
        return nil
}

func (p *parser) parseGoStmt() ast.Stmt {
        if p.trace {
                defer un(trace(p, "GoStmt"))
        }

        pos := p.expect(token.GO)
        call := p.parseCallExpr("go")
        p.expectSemi()
        if call == nil {
                return &ast.BadStmt{From: pos, To: pos + 2} // len("go")
        }

        return &ast.GoStmt{Go: pos, Call: call}
}

func (p *parser) parseDeferStmt() ast.Stmt {
        if p.trace {
                defer un(trace(p, "DeferStmt"))
        }

        pos := p.expect(token.DEFER)
        call := p.parseCallExpr("defer")
        p.expectSemi()
        if call == nil {
                return &ast.BadStmt{From: pos, To: pos + 5} // len("defer")
        }

        return &ast.DeferStmt{Defer: pos, Call: call}
}

func (p *parser) parseReturnStmt() *ast.ReturnStmt {
        if p.trace {
                defer un(trace(p, "ReturnStmt"))
        }

        pos := p.pos
        p.expect(token.RETURN)
        var x []ast.Expr
        if p.tok != token.SEMICOLON && p.tok != token.RBRACE {
                x = p.parseRhsList()
        }
        p.expectSemi()

        return &ast.ReturnStmt{Return: pos, Results: x}
}

func (p *parser) parseBranchStmt(tok token.Token) *ast.BranchStmt {
        if p.trace {
                defer un(trace(p, "BranchStmt"))
        }

        pos := p.expect(tok)
        var label *ast.Ident
        if tok != token.FALLTHROUGH && p.tok == token.IDENT {
                label = p.parseIdent()
                // add to list of unresolved targets
                n := len(p.targetStack) - 1
                p.targetStack[n] = append(p.targetStack[n], label)
        }
        p.expectSemi()

        return &ast.BranchStmt{TokPos: pos, Tok: tok, Label: label}
}

func (p *parser) makeExpr(s ast.Stmt, want string) ast.Expr {
        if s == nil {
                return nil
        }
        if es, isExpr := s.(*ast.ExprStmt); isExpr {
                return p.checkExpr(es.X)
        }
        found := "simple statement"
        if _, isAss := s.(*ast.AssignStmt); isAss {
                found = "assignment"
        }
        p.error(s.Pos(), fmt.Sprintf("expected %s, found %s (missing parentheses around composite literal?)", want, found))
        return &ast.BadExpr{From: s.Pos(), To: p.safePos(s.End())}
}

// parseIfHeader is an adjusted version of parser.header
// in cmd/compile/internal/syntax/parser.go, which has
// been tuned for better error handling.
func (p *parser) parseIfHeader() (init ast.Stmt, cond ast.Expr) {
        if p.tok == token.LBRACE {
                p.error(p.pos, "missing condition in if statement")
                cond = &ast.BadExpr{From: p.pos, To: p.pos}
                return
        }
        // p.tok != token.LBRACE

        outer := p.exprLev
        p.exprLev = -1

        if p.tok != token.SEMICOLON {
                // accept potential variable declaration but complain
                if p.tok == token.VAR {
                        p.next()
                        p.error(p.pos, fmt.Sprintf("var declaration not allowed in 'IF' initializer"))
                }
                init, _ = p.parseSimpleStmt(basic)
        }

        var condStmt ast.Stmt
        var semi struct {
                pos token.Pos
                lit string // ";" or "\n"; valid if pos.IsValid()
        }
        if p.tok != token.LBRACE {
                if p.tok == token.SEMICOLON {
                        semi.pos = p.pos
                        semi.lit = p.lit
                        p.next()
                } else {
                        p.expect(token.SEMICOLON)
                }
                if p.tok != token.LBRACE {
                        condStmt, _ = p.parseSimpleStmt(basic)
                }
        } else {
                condStmt = init
                init = nil
        }

        if condStmt != nil {
                cond = p.makeExpr(condStmt, "boolean expression")
        } else if semi.pos.IsValid() {
                if semi.lit == "\n" {
                        p.error(semi.pos, "unexpected newline, expecting { after if clause")
                } else {
                        p.error(semi.pos, "missing condition in if statement")
                }
        }

        // make sure we have a valid AST
        if cond == nil {
                cond = &ast.BadExpr{From: p.pos, To: p.pos}
        }

        p.exprLev = outer
        return
}

func (p *parser) parseIfStmt() *ast.IfStmt {
        if p.trace {
                defer un(trace(p, "IfStmt"))
        }

        pos := p.expect(token.IF)
        p.openScope()
        defer p.closeScope()

        init, cond := p.parseIfHeader()
        body := p.parseBlockStmt()

        var else_ ast.Stmt
        if p.tok == token.ELSE {
                p.next()
                switch p.tok {
                case token.IF:
                        else_ = p.parseIfStmt()
                case token.LBRACE:
                        else_ = p.parseBlockStmt()
                        p.expectSemi()
                default:
                        p.errorExpected(p.pos, "if statement or block")
                        else_ = &ast.BadStmt{From: p.pos, To: p.pos}
                }
        } else {
                p.expectSemi()
        }

        return &ast.IfStmt{If: pos, Init: init, Cond: cond, Body: body, Else: else_}
}

func (p *parser) parseTypeList() (list []ast.Expr) {
        if p.trace {
                defer un(trace(p, "TypeList"))
        }

        list = append(list, p.parseType())
        for p.tok == token.COMMA {
                p.next()
                list = append(list, p.parseType())
        }

        return
}

func (p *parser) parseCaseClause(typeSwitch bool) *ast.CaseClause {
        if p.trace {
                defer un(trace(p, "CaseClause"))
        }

        pos := p.pos
        var list []ast.Expr
        if p.tok == token.CASE {
                p.next()
                if typeSwitch {
                        list = p.parseTypeList()
                } else {
                        list = p.parseRhsList()
                }
        } else {
                p.expect(token.DEFAULT)
        }

        colon := p.expect(token.COLON)
        p.openScope()
        body := p.parseStmtList()
        p.closeScope()

        return &ast.CaseClause{Case: pos, List: list, Colon: colon, Body: body}
}

func isTypeSwitchAssert(x ast.Expr) bool {
        a, ok := x.(*ast.TypeAssertExpr)
        return ok && a.Type == nil
}

func (p *parser) isTypeSwitchGuard(s ast.Stmt) bool {
        switch t := s.(type) {
        case *ast.ExprStmt:
                // x.(type)
                return isTypeSwitchAssert(t.X)
        case *ast.AssignStmt:
                // v := x.(type)
                if len(t.Lhs) == 1 && len(t.Rhs) == 1 && isTypeSwitchAssert(t.Rhs[0]) {
                        switch t.Tok {
                        case token.ASSIGN:
                                // permit v = x.(type) but complain
                                p.error(t.TokPos, "expected ':=', found '='")
                                fallthrough
                        case token.DEFINE:
                                return true
                        }
                }
        }
        return false
}

func (p *parser) parseSwitchStmt() ast.Stmt {
        if p.trace {
                defer un(trace(p, "SwitchStmt"))
        }

        pos := p.expect(token.SWITCH)
        p.openScope()
        defer p.closeScope()

        var s1, s2 ast.Stmt
        if p.tok != token.LBRACE {
                prevLev := p.exprLev
                p.exprLev = -1
                if p.tok != token.SEMICOLON {
                        s2, _ = p.parseSimpleStmt(basic)
                }
                if p.tok == token.SEMICOLON {
                        p.next()
                        s1 = s2
                        s2 = nil
                        if p.tok != token.LBRACE {
                                // A TypeSwitchGuard may declare a variable in addition
                                // to the variable declared in the initial SimpleStmt.
                                // Introduce extra scope to avoid redeclaration errors:
                                //
                                //      switch t := 0; t := x.(T) { ... }
                                //
                                // (this code is not valid Go because the first t
                                // cannot be accessed and thus is never used, the extra
                                // scope is needed for the correct error message).
                                //
                                // If we don't have a type switch, s2 must be an expression.
                                // Having the extra nested but empty scope won't affect it.
                                p.openScope()
                                defer p.closeScope()
                                s2, _ = p.parseSimpleStmt(basic)
                        }
                }
                p.exprLev = prevLev
        }

        typeSwitch := p.isTypeSwitchGuard(s2)
        lbrace := p.expect(token.LBRACE)
        var list []ast.Stmt
        for p.tok == token.CASE || p.tok == token.DEFAULT {
                list = append(list, p.parseCaseClause(typeSwitch))
        }
        rbrace := p.expect(token.RBRACE)
        p.expectSemi()
        body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}

        if typeSwitch {
                return &ast.TypeSwitchStmt{Switch: pos, Init: s1, Assign: s2, Body: body}
        }

        return &ast.SwitchStmt{Switch: pos, Init: s1, Tag: p.makeExpr(s2, "switch expression"), Body: body}
}

func (p *parser) parseCommClause() *ast.CommClause {
        if p.trace {
                defer un(trace(p, "CommClause"))
        }

        p.openScope()
        pos := p.pos
        var comm ast.Stmt
        if p.tok == token.CASE {
                p.next()
                lhs := p.parseLhsList()
                if p.tok == token.ARROW {
                        // SendStmt
                        if len(lhs) > 1 {
                                p.errorExpected(lhs[0].Pos(), "1 expression")
                                // continue with first expression
                        }
                        arrow := p.pos
                        p.next()
                        rhs := p.parseRhs()
                        comm = &ast.SendStmt{Chan: lhs[0], Arrow: arrow, Value: rhs}
                } else {
                        // RecvStmt
                        if tok := p.tok; tok == token.ASSIGN || tok == token.DEFINE {
                                // RecvStmt with assignment
                                if len(lhs) > 2 {
                                        p.errorExpected(lhs[0].Pos(), "1 or 2 expressions")
                                        // continue with first two expressions
                                        lhs = lhs[0:2]
                                }
                                pos := p.pos
                                p.next()
                                rhs := p.parseRhs()
                                as := &ast.AssignStmt{Lhs: lhs, TokPos: pos, Tok: tok, Rhs: []ast.Expr{rhs}}
                                if tok == token.DEFINE {
                                        p.shortVarDecl(as, lhs)
                                }
                                comm = as
                        } else {
                                // lhs must be single receive operation
                                if len(lhs) > 1 {
                                        p.errorExpected(lhs[0].Pos(), "1 expression")
                                        // continue with first expression
                                }
                                comm = &ast.ExprStmt{X: lhs[0]}
                        }
                }
        } else {
                p.expect(token.DEFAULT)
        }

        colon := p.expect(token.COLON)
        body := p.parseStmtList()
        p.closeScope()

        return &ast.CommClause{Case: pos, Comm: comm, Colon: colon, Body: body}
}

func (p *parser) parseSelectStmt() *ast.SelectStmt {
        if p.trace {
                defer un(trace(p, "SelectStmt"))
        }

        pos := p.expect(token.SELECT)
        lbrace := p.expect(token.LBRACE)
        var list []ast.Stmt
        for p.tok == token.CASE || p.tok == token.DEFAULT {
                list = append(list, p.parseCommClause())
        }
        rbrace := p.expect(token.RBRACE)
        p.expectSemi()
        body := &ast.BlockStmt{Lbrace: lbrace, List: list, Rbrace: rbrace}

        return &ast.SelectStmt{Select: pos, Body: body}
}

func (p *parser) parseForStmt() ast.Stmt {
        if p.trace {
                defer un(trace(p, "ForStmt"))
        }

        pos := p.expect(token.FOR)
        p.openScope()
        defer p.closeScope()

        var s1, s2, s3 ast.Stmt
        var isRange bool
        if p.tok != token.LBRACE {
                prevLev := p.exprLev
                p.exprLev = -1
                if p.tok != token.SEMICOLON {
                        if p.tok == token.RANGE {
                                // "for range x" (nil lhs in assignment)
                                pos := p.pos
                                p.next()
                                y := []ast.Expr{&ast.UnaryExpr{OpPos: pos, Op: token.RANGE, X: p.parseRhs()}}
                                s2 = &ast.AssignStmt{Rhs: y}
                                isRange = true
                        } else {
                                s2, isRange = p.parseSimpleStmt(rangeOk)
                        }
                }
                if !isRange && p.tok == token.SEMICOLON {
                        p.next()
                        s1 = s2
                        s2 = nil
                        if p.tok != token.SEMICOLON {
                                s2, _ = p.parseSimpleStmt(basic)
                        }
                        p.expectSemi()
                        if p.tok != token.LBRACE {
                                s3, _ = p.parseSimpleStmt(basic)
                        }
                }
                p.exprLev = prevLev
        }

        body := p.parseBlockStmt()
        p.expectSemi()

        if isRange {
                as := s2.(*ast.AssignStmt)
                // check lhs
                var key, value ast.Expr
                switch len(as.Lhs) {
                case 0:
                        // nothing to do
                case 1:
                        key = as.Lhs[0]
                case 2:
                        key, value = as.Lhs[0], as.Lhs[1]
                default:
                        p.errorExpected(as.Lhs[len(as.Lhs)-1].Pos(), "at most 2 expressions")
                        return &ast.BadStmt{From: pos, To: p.safePos(body.End())}
                }
                // parseSimpleStmt returned a right-hand side that
                // is a single unary expression of the form "range x"
                x := as.Rhs[0].(*ast.UnaryExpr).X
                return &ast.RangeStmt{
                        For:    pos,
                        Key:    key,
                        Value:  value,
                        TokPos: as.TokPos,
                        Tok:    as.Tok,
                        X:      x,
                        Body:   body,
                }
        }

        // regular for statement
        return &ast.ForStmt{
                For:  pos,
                Init: s1,
                Cond: p.makeExpr(s2, "boolean or range expression"),
                Post: s3,
                Body: body,
        }
}

func (p *parser) parseStmt() (s ast.Stmt) {
        if p.trace {
                defer un(trace(p, "Statement"))
        }

        switch p.tok {
        case token.CONST, token.TYPE, token.VAR:
                s = &ast.DeclStmt{Decl: p.parseDecl(stmtStart)}
        case
                // tokens that may start an expression
                token.IDENT, token.INT, token.FLOAT, token.IMAG, token.CHAR, token.STRING, token.FUNC, token.LPAREN, // operands
                token.LBRACK, token.STRUCT, token.MAP, token.CHAN, token.INTERFACE, // composite types
                token.ADD, token.SUB, token.MUL, token.AND, token.XOR, token.ARROW, token.NOT: // unary operators
                s, _ = p.parseSimpleStmt(labelOk)
                // because of the required look-ahead, labeled statements are
                // parsed by parseSimpleStmt - don't expect a semicolon after
                // them
                if _, isLabeledStmt := s.(*ast.LabeledStmt); !isLabeledStmt {
                        p.expectSemi()
                }
        case token.GO:
                s = p.parseGoStmt()
        case token.DEFER:
                s = p.parseDeferStmt()
        case token.RETURN:
                s = p.parseReturnStmt()
        case token.BREAK, token.CONTINUE, token.GOTO, token.FALLTHROUGH:
                s = p.parseBranchStmt(p.tok)
        case token.LBRACE:
                s = p.parseBlockStmt()
                p.expectSemi()
        case token.IF:
                s = p.parseIfStmt()
        case token.SWITCH:
                s = p.parseSwitchStmt()
        case token.SELECT:
                s = p.parseSelectStmt()
        case token.FOR:
                s = p.parseForStmt()
        case token.SEMICOLON:
                // Is it ever possible to have an implicit semicolon
                // producing an empty statement in a valid program?
                // (handle correctly anyway)
                s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: p.lit == "\n"}
                p.next()
        case token.RBRACE:
                // a semicolon may be omitted before a closing "}"
                s = &ast.EmptyStmt{Semicolon: p.pos, Implicit: true}
        default:
                // no statement found
                pos := p.pos
                p.errorExpected(pos, "statement")
                p.advance(stmtStart)
                s = &ast.BadStmt{From: pos, To: p.pos}
        }

        return
}

// ----------------------------------------------------------------------------
// Declarations

type parseSpecFunction func(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec

func isValidImport(lit string) bool {
        const illegalChars = `!"#$%&'()*,:;<=>?[\]^{|}` + "`\uFFFD"
        s, _ := strconv.Unquote(lit) // go/scanner returns a legal string literal
        for _, r := range s {
                if !unicode.IsGraphic(r) || unicode.IsSpace(r) || strings.ContainsRune(illegalChars, r) {
                        return false
                }
        }
        return s != ""
}

func (p *parser) parseImportSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
        if p.trace {
                defer un(trace(p, "ImportSpec"))
        }

        var ident *ast.Ident
        switch p.tok {
        case token.PERIOD:
                ident = &ast.Ident{NamePos: p.pos, Name: "."}
                p.next()
        case token.IDENT:
                ident = p.parseIdent()
        }

        pos := p.pos
        var path string
        if p.tok == token.STRING {
                path = p.lit
                if !isValidImport(path) {
                        p.error(pos, "invalid import path: "+path)
                }
                p.next()
        } else {
                p.expect(token.STRING) // use expect() error handling
        }
        p.expectSemi() // call before accessing p.linecomment

        // collect imports
        spec := &ast.ImportSpec{
                Doc:     doc,
                Name:    ident,
                Path:    &ast.BasicLit{ValuePos: pos, Kind: token.STRING, Value: path},
                Comment: p.lineComment,
        }
        p.imports = append(p.imports, spec)

        return spec
}

func (p *parser) parseValueSpec(doc *ast.CommentGroup, keyword token.Token, iota int) ast.Spec {
        if p.trace {
                defer un(trace(p, keyword.String()+"Spec"))
        }

        pos := p.pos
        idents := p.parseIdentList()
        typ := p.tryType()
        var values []ast.Expr
        // always permit optional initialization for more tolerant parsing
        if p.tok == token.ASSIGN {
                p.next()
                values = p.parseRhsList()
        }
        p.expectSemi() // call before accessing p.linecomment

        switch keyword {
        case token.VAR:
                if typ == nil && values == nil {
                        p.error(pos, "missing variable type or initialization")
                }
        case token.CONST:
                if values == nil && (iota == 0 || typ != nil) {
                        p.error(pos, "missing constant value")
                }
        }

        // Go spec: The scope of a constant or variable identifier declared inside
        // a function begins at the end of the ConstSpec or VarSpec and ends at
        // the end of the innermost containing block.
        // (Global identifiers are resolved in a separate phase after parsing.)
        spec := &ast.ValueSpec{
                Doc:     doc,
                Names:   idents,
                Type:    typ,
                Values:  values,
                Comment: p.lineComment,
        }
        kind := ast.Con
        if keyword == token.VAR {
                kind = ast.Var
        }
        p.declare(spec, iota, p.topScope, kind, idents...)

        return spec
}

func (p *parser) parseTypeSpec(doc *ast.CommentGroup, _ token.Token, _ int) ast.Spec {
        if p.trace {
                defer un(trace(p, "TypeSpec"))
        }

        ident := p.parseIdent()

        // Go spec: The scope of a type identifier declared inside a function begins
        // at the identifier in the TypeSpec and ends at the end of the innermost
        // containing block.
        // (Global identifiers are resolved in a separate phase after parsing.)
        spec := &ast.TypeSpec{Doc: doc, Name: ident}
        p.declare(spec, nil, p.topScope, ast.Typ, ident)
        if p.tok == token.ASSIGN {
                spec.Assign = p.pos
                p.next()
        }
        spec.Type = p.parseType()
        p.expectSemi() // call before accessing p.linecomment
        spec.Comment = p.lineComment

        return spec
}

func (p *parser) parseGenDecl(keyword token.Token, f parseSpecFunction) *ast.GenDecl {
        if p.trace {
                defer un(trace(p, "GenDecl("+keyword.String()+")"))
        }

        doc := p.leadComment
        pos := p.expect(keyword)
        var lparen, rparen token.Pos
        var list []ast.Spec
        if p.tok == token.LPAREN {
                lparen = p.pos
                p.next()
                for iota := 0; p.tok != token.RPAREN && p.tok != token.EOF; iota++ {
                        list = append(list, f(p.leadComment, keyword, iota))
                }
                rparen = p.expect(token.RPAREN)
                p.expectSemi()
        } else {
                list = append(list, f(nil, keyword, 0))
        }

        return &ast.GenDecl{
                Doc:    doc,
                TokPos: pos,
                Tok:    keyword,
                Lparen: lparen,
                Specs:  list,
                Rparen: rparen,
        }
}

func (p *parser) parseFuncDecl() *ast.FuncDecl {
        if p.trace {
                defer un(trace(p, "FunctionDecl"))
        }

        doc := p.leadComment
        pos := p.expect(token.FUNC)
        scope := ast.NewScope(p.topScope) // function scope

        var recv *ast.FieldList
        if p.tok == token.LPAREN {
                recv = p.parseParameters(scope, false)
        }

        ident := p.parseIdent()

        params, results := p.parseSignature(scope)

        var body *ast.BlockStmt
        if p.tok == token.LBRACE {
                body = p.parseBody(scope)
        }
        p.expectSemi()

        decl := &ast.FuncDecl{
                Doc:  doc,
                Recv: recv,
                Name: ident,
                Type: &ast.FuncType{
                        Func:    pos,
                        Params:  params,
                        Results: results,
                },
                Body: body,
        }
        if recv == nil {
                // Go spec: The scope of an identifier denoting a constant, type,
                // variable, or function (but not method) declared at top level
                // (outside any function) is the package block.
                //
                // init() functions cannot be referred to and there may
                // be more than one - don't put them in the pkgScope
                if ident.Name != "init" {
                        p.declare(decl, nil, p.pkgScope, ast.Fun, ident)
                }
        }

        return decl
}

func (p *parser) parseDecl(sync map[token.Token]bool) ast.Decl {
        if p.trace {
                defer un(trace(p, "Declaration"))
        }

        var f parseSpecFunction
        switch p.tok {
        case token.CONST, token.VAR:
                f = p.parseValueSpec

        case token.TYPE:
                f = p.parseTypeSpec

        case token.FUNC:
                return p.parseFuncDecl()

        default:
                pos := p.pos
                p.errorExpected(pos, "declaration")
                p.advance(sync)
                return &ast.BadDecl{From: pos, To: p.pos}
        }

        return p.parseGenDecl(p.tok, f)
}

// ----------------------------------------------------------------------------
// Source files

func (p *parser) parseFile() *ast.File {
        if p.trace {
                defer un(trace(p, "File"))
        }

        // Don't bother parsing the rest if we had errors scanning the first token.
        // Likely not a Go source file at all.
        if p.errors.Len() != 0 {
                return nil
        }

        // package clause
        doc := p.leadComment
        pos := p.expect(token.PACKAGE)
        // Go spec: The package clause is not a declaration;
        // the package name does not appear in any scope.
        ident := p.parseIdent()
        if ident.Name == "_" && p.mode&DeclarationErrors != 0 {
                p.error(p.pos, "invalid package name _")
        }
        p.expectSemi()

        // Don't bother parsing the rest if we had errors parsing the package clause.
        // Likely not a Go source file at all.
        if p.errors.Len() != 0 {
                return nil
        }

        p.openScope()
        p.pkgScope = p.topScope
        var decls []ast.Decl
        if p.mode&PackageClauseOnly == 0 {
                // import decls
                for p.tok == token.IMPORT {
                        decls = append(decls, p.parseGenDecl(token.IMPORT, p.parseImportSpec))
                }

                if p.mode&ImportsOnly == 0 {
                        // rest of package body
                        for p.tok != token.EOF {
                                decls = append(decls, p.parseDecl(declStart))
                        }
                }
        }
        p.closeScope()
        assert(p.topScope == nil, "unbalanced scopes")
        assert(p.labelScope == nil, "unbalanced label scopes")

        // resolve global identifiers within the same file
        i := 0
        for _, ident := range p.unresolved {
                // i <= index for current ident
                assert(ident.Obj == unresolved, "object already resolved")
                ident.Obj = p.pkgScope.Lookup(ident.Name) // also removes unresolved sentinel
                if ident.Obj == nil {
                        p.unresolved[i] = ident
                        i++
                }
        }

        return &ast.File{
                Doc:        doc,
                Package:    pos,
                Name:       ident,
                Decls:      decls,
                Scope:      p.pkgScope,
                Imports:    p.imports,
                Unresolved: p.unresolved[0:i],
                Comments:   p.comments,
        }
}