Daily bump.

[official-gcc.git] / libgo / go / text / template / exec.go

// Copyright 2011 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 template

import (
        "errors"
        "fmt"
        "internal/fmtsort"
        "io"
        "reflect"
        "runtime"
        "strings"
        "text/template/parse"
)

// maxExecDepth specifies the maximum stack depth of templates within
// templates. This limit is only practically reached by accidentally
// recursive template invocations. This limit allows us to return
// an error instead of triggering a stack overflow.
var maxExecDepth = initMaxExecDepth()

func initMaxExecDepth() int {
        // For gccgo we make this 1000 rather than 100000 to avoid
        // stack overflow on non-split-stack systems.
        if runtime.GOARCH == "wasm" || runtime.Compiler == "gccgo" {
                return 1000
        }
        return 100000
}

// state represents the state of an execution. It's not part of the
// template so that multiple executions of the same template
// can execute in parallel.
type state struct {
        tmpl  *Template
        wr    io.Writer
        node  parse.Node // current node, for errors
        vars  []variable // push-down stack of variable values.
        depth int        // the height of the stack of executing templates.
}

// variable holds the dynamic value of a variable such as $, $x etc.
type variable struct {
        name  string
        value reflect.Value
}

// push pushes a new variable on the stack.
func (s *state) push(name string, value reflect.Value) {
        s.vars = append(s.vars, variable{name, value})
}

// mark returns the length of the variable stack.
func (s *state) mark() int {
        return len(s.vars)
}

// pop pops the variable stack up to the mark.
func (s *state) pop(mark int) {
        s.vars = s.vars[0:mark]
}

// setVar overwrites the last declared variable with the given name.
// Used by variable assignments.
func (s *state) setVar(name string, value reflect.Value) {
        for i := s.mark() - 1; i >= 0; i-- {
                if s.vars[i].name == name {
                        s.vars[i].value = value
                        return
                }
        }
        s.errorf("undefined variable: %s", name)
}

// setTopVar overwrites the top-nth variable on the stack. Used by range iterations.
func (s *state) setTopVar(n int, value reflect.Value) {
        s.vars[len(s.vars)-n].value = value
}

// varValue returns the value of the named variable.
func (s *state) varValue(name string) reflect.Value {
        for i := s.mark() - 1; i >= 0; i-- {
                if s.vars[i].name == name {
                        return s.vars[i].value
                }
        }
        s.errorf("undefined variable: %s", name)
        return zero
}

var zero reflect.Value

type missingValType struct{}

var missingVal = reflect.ValueOf(missingValType{})

// at marks the state to be on node n, for error reporting.
func (s *state) at(node parse.Node) {
        s.node = node
}

// doublePercent returns the string with %'s replaced by %%, if necessary,
// so it can be used safely inside a Printf format string.
func doublePercent(str string) string {
        return strings.ReplaceAll(str, "%", "%%")
}

// TODO: It would be nice if ExecError was more broken down, but
// the way ErrorContext embeds the template name makes the
// processing too clumsy.

// ExecError is the custom error type returned when Execute has an
// error evaluating its template. (If a write error occurs, the actual
// error is returned; it will not be of type ExecError.)
type ExecError struct {
        Name string // Name of template.
        Err  error  // Pre-formatted error.
}

func (e ExecError) Error() string {
        return e.Err.Error()
}

func (e ExecError) Unwrap() error {
        return e.Err
}

// errorf records an ExecError and terminates processing.
func (s *state) errorf(format string, args ...any) {
        name := doublePercent(s.tmpl.Name())
        if s.node == nil {
                format = fmt.Sprintf("template: %s: %s", name, format)
        } else {
                location, context := s.tmpl.ErrorContext(s.node)
                format = fmt.Sprintf("template: %s: executing %q at <%s>: %s", location, name, doublePercent(context), format)
        }
        panic(ExecError{
                Name: s.tmpl.Name(),
                Err:  fmt.Errorf(format, args...),
        })
}

// writeError is the wrapper type used internally when Execute has an
// error writing to its output. We strip the wrapper in errRecover.
// Note that this is not an implementation of error, so it cannot escape
// from the package as an error value.
type writeError struct {
        Err error // Original error.
}

func (s *state) writeError(err error) {
        panic(writeError{
                Err: err,
        })
}

// errRecover is the handler that turns panics into returns from the top
// level of Parse.
func errRecover(errp *error) {
        e := recover()
        if e != nil {
                switch err := e.(type) {
                case runtime.Error:
                        panic(e)
                case writeError:
                        *errp = err.Err // Strip the wrapper.
                case ExecError:
                        *errp = err // Keep the wrapper.
                default:
                        panic(e)
                }
        }
}

// ExecuteTemplate applies the template associated with t that has the given name
// to the specified data object and writes the output to wr.
// If an error occurs executing the template or writing its output,
// execution stops, but partial results may already have been written to
// the output writer.
// A template may be executed safely in parallel, although if parallel
// executions share a Writer the output may be interleaved.
func (t *Template) ExecuteTemplate(wr io.Writer, name string, data any) error {
        tmpl := t.Lookup(name)
        if tmpl == nil {
                return fmt.Errorf("template: no template %q associated with template %q", name, t.name)
        }
        return tmpl.Execute(wr, data)
}

// Execute applies a parsed template to the specified data object,
// and writes the output to wr.
// If an error occurs executing the template or writing its output,
// execution stops, but partial results may already have been written to
// the output writer.
// A template may be executed safely in parallel, although if parallel
// executions share a Writer the output may be interleaved.
//
// If data is a reflect.Value, the template applies to the concrete
// value that the reflect.Value holds, as in fmt.Print.
func (t *Template) Execute(wr io.Writer, data any) error {
        return t.execute(wr, data)
}

func (t *Template) execute(wr io.Writer, data any) (err error) {
        defer errRecover(&err)
        value, ok := data.(reflect.Value)
        if !ok {
                value = reflect.ValueOf(data)
        }
        state := &state{
                tmpl: t,
                wr:   wr,
                vars: []variable{{"$", value}},
        }
        if t.Tree == nil || t.Root == nil {
                state.errorf("%q is an incomplete or empty template", t.Name())
        }
        state.walk(value, t.Root)
        return
}

// DefinedTemplates returns a string listing the defined templates,
// prefixed by the string "; defined templates are: ". If there are none,
// it returns the empty string. For generating an error message here
// and in html/template.
func (t *Template) DefinedTemplates() string {
        if t.common == nil {
                return ""
        }
        var b strings.Builder
        t.muTmpl.RLock()
        defer t.muTmpl.RUnlock()
        for name, tmpl := range t.tmpl {
                if tmpl.Tree == nil || tmpl.Root == nil {
                        continue
                }
                if b.Len() == 0 {
                        b.WriteString("; defined templates are: ")
                } else {
                        b.WriteString(", ")
                }
                fmt.Fprintf(&b, "%q", name)
        }
        return b.String()
}

// Sentinel errors for use with panic to signal early exits from range loops.
var (
        walkBreak    = errors.New("break")
        walkContinue = errors.New("continue")
)

// Walk functions step through the major pieces of the template structure,
// generating output as they go.
func (s *state) walk(dot reflect.Value, node parse.Node) {
        s.at(node)
        switch node := node.(type) {
        case *parse.ActionNode:
                // Do not pop variables so they persist until next end.
                // Also, if the action declares variables, don't print the result.
                val := s.evalPipeline(dot, node.Pipe)
                if len(node.Pipe.Decl) == 0 {
                        s.printValue(node, val)
                }
        case *parse.BreakNode:
                panic(walkBreak)
        case *parse.CommentNode:
        case *parse.ContinueNode:
                panic(walkContinue)
        case *parse.IfNode:
                s.walkIfOrWith(parse.NodeIf, dot, node.Pipe, node.List, node.ElseList)
        case *parse.ListNode:
                for _, node := range node.Nodes {
                        s.walk(dot, node)
                }
        case *parse.RangeNode:
                s.walkRange(dot, node)
        case *parse.TemplateNode:
                s.walkTemplate(dot, node)
        case *parse.TextNode:
                if _, err := s.wr.Write(node.Text); err != nil {
                        s.writeError(err)
                }
        case *parse.WithNode:
                s.walkIfOrWith(parse.NodeWith, dot, node.Pipe, node.List, node.ElseList)
        default:
                s.errorf("unknown node: %s", node)
        }
}

// walkIfOrWith walks an 'if' or 'with' node. The two control structures
// are identical in behavior except that 'with' sets dot.
func (s *state) walkIfOrWith(typ parse.NodeType, dot reflect.Value, pipe *parse.PipeNode, list, elseList *parse.ListNode) {
        defer s.pop(s.mark())
        val := s.evalPipeline(dot, pipe)
        truth, ok := isTrue(indirectInterface(val))
        if !ok {
                s.errorf("if/with can't use %v", val)
        }
        if truth {
                if typ == parse.NodeWith {
                        s.walk(val, list)
                } else {
                        s.walk(dot, list)
                }
        } else if elseList != nil {
                s.walk(dot, elseList)
        }
}

// IsTrue reports whether the value is 'true', in the sense of not the zero of its type,
// and whether the value has a meaningful truth value. This is the definition of
// truth used by if and other such actions.
func IsTrue(val any) (truth, ok bool) {
        return isTrue(reflect.ValueOf(val))
}

func isTrue(val reflect.Value) (truth, ok bool) {
        if !val.IsValid() {
                // Something like var x interface{}, never set. It's a form of nil.
                return false, true
        }
        switch val.Kind() {
        case reflect.Array, reflect.Map, reflect.Slice, reflect.String:
                truth = val.Len() > 0
        case reflect.Bool:
                truth = val.Bool()
        case reflect.Complex64, reflect.Complex128:
                truth = val.Complex() != 0
        case reflect.Chan, reflect.Func, reflect.Pointer, reflect.Interface:
                truth = !val.IsNil()
        case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
                truth = val.Int() != 0
        case reflect.Float32, reflect.Float64:
                truth = val.Float() != 0
        case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
                truth = val.Uint() != 0
        case reflect.Struct:
                truth = true // Struct values are always true.
        default:
                return
        }
        return truth, true
}

func (s *state) walkRange(dot reflect.Value, r *parse.RangeNode) {
        s.at(r)
        defer func() {
                if r := recover(); r != nil && r != walkBreak {
                        panic(r)
                }
        }()
        defer s.pop(s.mark())
        val, _ := indirect(s.evalPipeline(dot, r.Pipe))
        // mark top of stack before any variables in the body are pushed.
        mark := s.mark()
        oneIteration := func(index, elem reflect.Value) {
                // Set top var (lexically the second if there are two) to the element.
                if len(r.Pipe.Decl) > 0 {
                        s.setTopVar(1, elem)
                }
                // Set next var (lexically the first if there are two) to the index.
                if len(r.Pipe.Decl) > 1 {
                        s.setTopVar(2, index)
                }
                defer s.pop(mark)
                defer func() {
                        // Consume panic(walkContinue)
                        if r := recover(); r != nil && r != walkContinue {
                                panic(r)
                        }
                }()
                s.walk(elem, r.List)
        }
        switch val.Kind() {
        case reflect.Array, reflect.Slice:
                if val.Len() == 0 {
                        break
                }
                for i := 0; i < val.Len(); i++ {
                        oneIteration(reflect.ValueOf(i), val.Index(i))
                }
                return
        case reflect.Map:
                if val.Len() == 0 {
                        break
                }
                om := fmtsort.Sort(val)
                for i, key := range om.Key {
                        oneIteration(key, om.Value[i])
                }
                return
        case reflect.Chan:
                if val.IsNil() {
                        break
                }
                if val.Type().ChanDir() == reflect.SendDir {
                        s.errorf("range over send-only channel %v", val)
                        break
                }
                i := 0
                for ; ; i++ {
                        elem, ok := val.Recv()
                        if !ok {
                                break
                        }
                        oneIteration(reflect.ValueOf(i), elem)
                }
                if i == 0 {
                        break
                }
                return
        case reflect.Invalid:
                break // An invalid value is likely a nil map, etc. and acts like an empty map.
        default:
                s.errorf("range can't iterate over %v", val)
        }
        if r.ElseList != nil {
                s.walk(dot, r.ElseList)
        }
}

func (s *state) walkTemplate(dot reflect.Value, t *parse.TemplateNode) {
        s.at(t)
        tmpl := s.tmpl.Lookup(t.Name)
        if tmpl == nil {
                s.errorf("template %q not defined", t.Name)
        }
        if s.depth == maxExecDepth {
                s.errorf("exceeded maximum template depth (%v)", maxExecDepth)
        }
        // Variables declared by the pipeline persist.
        dot = s.evalPipeline(dot, t.Pipe)
        newState := *s
        newState.depth++
        newState.tmpl = tmpl
        // No dynamic scoping: template invocations inherit no variables.
        newState.vars = []variable{{"$", dot}}
        newState.walk(dot, tmpl.Root)
}

// Eval functions evaluate pipelines, commands, and their elements and extract
// values from the data structure by examining fields, calling methods, and so on.
// The printing of those values happens only through walk functions.

// evalPipeline returns the value acquired by evaluating a pipeline. If the
// pipeline has a variable declaration, the variable will be pushed on the
// stack. Callers should therefore pop the stack after they are finished
// executing commands depending on the pipeline value.
func (s *state) evalPipeline(dot reflect.Value, pipe *parse.PipeNode) (value reflect.Value) {
        if pipe == nil {
                return
        }
        s.at(pipe)
        value = missingVal
        for _, cmd := range pipe.Cmds {
                value = s.evalCommand(dot, cmd, value) // previous value is this one's final arg.
                // If the object has type interface{}, dig down one level to the thing inside.
                if value.Kind() == reflect.Interface && value.Type().NumMethod() == 0 {
                        value = reflect.ValueOf(value.Interface()) // lovely!
                }
        }
        for _, variable := range pipe.Decl {
                if pipe.IsAssign {
                        s.setVar(variable.Ident[0], value)
                } else {
                        s.push(variable.Ident[0], value)
                }
        }
        return value
}

func (s *state) notAFunction(args []parse.Node, final reflect.Value) {
        if len(args) > 1 || final != missingVal {
                s.errorf("can't give argument to non-function %s", args[0])
        }
}

func (s *state) evalCommand(dot reflect.Value, cmd *parse.CommandNode, final reflect.Value) reflect.Value {
        firstWord := cmd.Args[0]
        switch n := firstWord.(type) {
        case *parse.FieldNode:
                return s.evalFieldNode(dot, n, cmd.Args, final)
        case *parse.ChainNode:
                return s.evalChainNode(dot, n, cmd.Args, final)
        case *parse.IdentifierNode:
                // Must be a function.
                return s.evalFunction(dot, n, cmd, cmd.Args, final)
        case *parse.PipeNode:
                // Parenthesized pipeline. The arguments are all inside the pipeline; final must be absent.
                s.notAFunction(cmd.Args, final)
                return s.evalPipeline(dot, n)
        case *parse.VariableNode:
                return s.evalVariableNode(dot, n, cmd.Args, final)
        }
        s.at(firstWord)
        s.notAFunction(cmd.Args, final)
        switch word := firstWord.(type) {
        case *parse.BoolNode:
                return reflect.ValueOf(word.True)
        case *parse.DotNode:
                return dot
        case *parse.NilNode:
                s.errorf("nil is not a command")
        case *parse.NumberNode:
                return s.idealConstant(word)
        case *parse.StringNode:
                return reflect.ValueOf(word.Text)
        }
        s.errorf("can't evaluate command %q", firstWord)
        panic("not reached")
}

// idealConstant is called to return the value of a number in a context where
// we don't know the type. In that case, the syntax of the number tells us
// its type, and we use Go rules to resolve. Note there is no such thing as
// a uint ideal constant in this situation - the value must be of int type.
func (s *state) idealConstant(constant *parse.NumberNode) reflect.Value {
        // These are ideal constants but we don't know the type
        // and we have no context.  (If it was a method argument,
        // we'd know what we need.) The syntax guides us to some extent.
        s.at(constant)
        switch {
        case constant.IsComplex:
                return reflect.ValueOf(constant.Complex128) // incontrovertible.

        case constant.IsFloat &&
                !isHexInt(constant.Text) && !isRuneInt(constant.Text) &&
                strings.ContainsAny(constant.Text, ".eEpP"):
                return reflect.ValueOf(constant.Float64)

        case constant.IsInt:
                n := int(constant.Int64)
                if int64(n) != constant.Int64 {
                        s.errorf("%s overflows int", constant.Text)
                }
                return reflect.ValueOf(n)

        case constant.IsUint:
                s.errorf("%s overflows int", constant.Text)
        }
        return zero
}

func isRuneInt(s string) bool {
        return len(s) > 0 && s[0] == '\''
}

func isHexInt(s string) bool {
        return len(s) > 2 && s[0] == '0' && (s[1] == 'x' || s[1] == 'X') && !strings.ContainsAny(s, "pP")
}

func (s *state) evalFieldNode(dot reflect.Value, field *parse.FieldNode, args []parse.Node, final reflect.Value) reflect.Value {
        s.at(field)
        return s.evalFieldChain(dot, dot, field, field.Ident, args, final)
}

func (s *state) evalChainNode(dot reflect.Value, chain *parse.ChainNode, args []parse.Node, final reflect.Value) reflect.Value {
        s.at(chain)
        if len(chain.Field) == 0 {
                s.errorf("internal error: no fields in evalChainNode")
        }
        if chain.Node.Type() == parse.NodeNil {
                s.errorf("indirection through explicit nil in %s", chain)
        }
        // (pipe).Field1.Field2 has pipe as .Node, fields as .Field. Eval the pipeline, then the fields.
        pipe := s.evalArg(dot, nil, chain.Node)
        return s.evalFieldChain(dot, pipe, chain, chain.Field, args, final)
}

func (s *state) evalVariableNode(dot reflect.Value, variable *parse.VariableNode, args []parse.Node, final reflect.Value) reflect.Value {
        // $x.Field has $x as the first ident, Field as the second. Eval the var, then the fields.
        s.at(variable)
        value := s.varValue(variable.Ident[0])
        if len(variable.Ident) == 1 {
                s.notAFunction(args, final)
                return value
        }
        return s.evalFieldChain(dot, value, variable, variable.Ident[1:], args, final)
}

// evalFieldChain evaluates .X.Y.Z possibly followed by arguments.
// dot is the environment in which to evaluate arguments, while
// receiver is the value being walked along the chain.
func (s *state) evalFieldChain(dot, receiver reflect.Value, node parse.Node, ident []string, args []parse.Node, final reflect.Value) reflect.Value {
        n := len(ident)
        for i := 0; i < n-1; i++ {
                receiver = s.evalField(dot, ident[i], node, nil, missingVal, receiver)
        }
        // Now if it's a method, it gets the arguments.
        return s.evalField(dot, ident[n-1], node, args, final, receiver)
}

func (s *state) evalFunction(dot reflect.Value, node *parse.IdentifierNode, cmd parse.Node, args []parse.Node, final reflect.Value) reflect.Value {
        s.at(node)
        name := node.Ident
        function, isBuiltin, ok := findFunction(name, s.tmpl)
        if !ok {
                s.errorf("%q is not a defined function", name)
        }
        return s.evalCall(dot, function, isBuiltin, cmd, name, args, final)
}

// evalField evaluates an expression like (.Field) or (.Field arg1 arg2).
// The 'final' argument represents the return value from the preceding
// value of the pipeline, if any.
func (s *state) evalField(dot reflect.Value, fieldName string, node parse.Node, args []parse.Node, final, receiver reflect.Value) reflect.Value {
        if !receiver.IsValid() {
                if s.tmpl.option.missingKey == mapError { // Treat invalid value as missing map key.
                        s.errorf("nil data; no entry for key %q", fieldName)
                }
                return zero
        }
        typ := receiver.Type()
        receiver, isNil := indirect(receiver)
        if receiver.Kind() == reflect.Interface && isNil {
                // Calling a method on a nil interface can't work. The
                // MethodByName method call below would panic.
                s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
                return zero
        }

        // Unless it's an interface, need to get to a value of type *T to guarantee
        // we see all methods of T and *T.
        ptr := receiver
        if ptr.Kind() != reflect.Interface && ptr.Kind() != reflect.Pointer && ptr.CanAddr() {
                ptr = ptr.Addr()
        }
        if method := ptr.MethodByName(fieldName); method.IsValid() {
                return s.evalCall(dot, method, false, node, fieldName, args, final)
        }
        hasArgs := len(args) > 1 || final != missingVal
        // It's not a method; must be a field of a struct or an element of a map.
        switch receiver.Kind() {
        case reflect.Struct:
                tField, ok := receiver.Type().FieldByName(fieldName)
                if ok {
                        field, err := receiver.FieldByIndexErr(tField.Index)
                        if !tField.IsExported() {
                                s.errorf("%s is an unexported field of struct type %s", fieldName, typ)
                        }
                        if err != nil {
                                s.errorf("%v", err)
                        }
                        // If it's a function, we must call it.
                        if hasArgs {
                                s.errorf("%s has arguments but cannot be invoked as function", fieldName)
                        }
                        return field
                }
        case reflect.Map:
                // If it's a map, attempt to use the field name as a key.
                nameVal := reflect.ValueOf(fieldName)
                if nameVal.Type().AssignableTo(receiver.Type().Key()) {
                        if hasArgs {
                                s.errorf("%s is not a method but has arguments", fieldName)
                        }
                        result := receiver.MapIndex(nameVal)
                        if !result.IsValid() {
                                switch s.tmpl.option.missingKey {
                                case mapInvalid:
                                        // Just use the invalid value.
                                case mapZeroValue:
                                        result = reflect.Zero(receiver.Type().Elem())
                                case mapError:
                                        s.errorf("map has no entry for key %q", fieldName)
                                }
                        }
                        return result
                }
        case reflect.Pointer:
                etyp := receiver.Type().Elem()
                if etyp.Kind() == reflect.Struct {
                        if _, ok := etyp.FieldByName(fieldName); !ok {
                                // If there's no such field, say "can't evaluate"
                                // instead of "nil pointer evaluating".
                                break
                        }
                }
                if isNil {
                        s.errorf("nil pointer evaluating %s.%s", typ, fieldName)
                }
        }
        s.errorf("can't evaluate field %s in type %s", fieldName, typ)
        panic("not reached")
}

var (
        errorType        = reflect.TypeOf((*error)(nil)).Elem()
        fmtStringerType  = reflect.TypeOf((*fmt.Stringer)(nil)).Elem()
        reflectValueType = reflect.TypeOf((*reflect.Value)(nil)).Elem()
)

// evalCall executes a function or method call. If it's a method, fun already has the receiver bound, so
// it looks just like a function call. The arg list, if non-nil, includes (in the manner of the shell), arg[0]
// as the function itself.
func (s *state) evalCall(dot, fun reflect.Value, isBuiltin bool, node parse.Node, name string, args []parse.Node, final reflect.Value) reflect.Value {
        if args != nil {
                args = args[1:] // Zeroth arg is function name/node; not passed to function.
        }
        typ := fun.Type()
        numIn := len(args)
        if final != missingVal {
                numIn++
        }
        numFixed := len(args)
        if typ.IsVariadic() {
                numFixed = typ.NumIn() - 1 // last arg is the variadic one.
                if numIn < numFixed {
                        s.errorf("wrong number of args for %s: want at least %d got %d", name, typ.NumIn()-1, len(args))
                }
        } else if numIn != typ.NumIn() {
                s.errorf("wrong number of args for %s: want %d got %d", name, typ.NumIn(), numIn)
        }
        if !goodFunc(typ) {
                // TODO: This could still be a confusing error; maybe goodFunc should provide info.
                s.errorf("can't call method/function %q with %d results", name, typ.NumOut())
        }

        unwrap := func(v reflect.Value) reflect.Value {
                if v.Type() == reflectValueType {
                        v = v.Interface().(reflect.Value)
                }
                return v
        }

        // Special case for builtin and/or, which short-circuit.
        if isBuiltin && (name == "and" || name == "or") {
                argType := typ.In(0)
                var v reflect.Value
                for _, arg := range args {
                        v = s.evalArg(dot, argType, arg).Interface().(reflect.Value)
                        if truth(v) == (name == "or") {
                                // This value was already unwrapped
                                // by the .Interface().(reflect.Value).
                                return v
                        }
                }
                if final != missingVal {
                        // The last argument to and/or is coming from
                        // the pipeline. We didn't short circuit on an earlier
                        // argument, so we are going to return this one.
                        // We don't have to evaluate final, but we do
                        // have to check its type. Then, since we are
                        // going to return it, we have to unwrap it.
                        v = unwrap(s.validateType(final, argType))
                }
                return v
        }

        // Build the arg list.
        argv := make([]reflect.Value, numIn)
        // Args must be evaluated. Fixed args first.
        i := 0
        for ; i < numFixed && i < len(args); i++ {
                argv[i] = s.evalArg(dot, typ.In(i), args[i])
        }
        // Now the ... args.
        if typ.IsVariadic() {
                argType := typ.In(typ.NumIn() - 1).Elem() // Argument is a slice.
                for ; i < len(args); i++ {
                        argv[i] = s.evalArg(dot, argType, args[i])
                }
        }
        // Add final value if necessary.
        if final != missingVal {
                t := typ.In(typ.NumIn() - 1)
                if typ.IsVariadic() {
                        if numIn-1 < numFixed {
                                // The added final argument corresponds to a fixed parameter of the function.
                                // Validate against the type of the actual parameter.
                                t = typ.In(numIn - 1)
                        } else {
                                // The added final argument corresponds to the variadic part.
                                // Validate against the type of the elements of the variadic slice.
                                t = t.Elem()
                        }
                }
                argv[i] = s.validateType(final, t)
        }
        v, err := safeCall(fun, argv)
        // If we have an error that is not nil, stop execution and return that
        // error to the caller.
        if err != nil {
                s.at(node)
                s.errorf("error calling %s: %w", name, err)
        }
        return unwrap(v)
}

// canBeNil reports whether an untyped nil can be assigned to the type. See reflect.Zero.
func canBeNil(typ reflect.Type) bool {
        switch typ.Kind() {
        case reflect.Chan, reflect.Func, reflect.Interface, reflect.Map, reflect.Pointer, reflect.Slice:
                return true
        case reflect.Struct:
                return typ == reflectValueType
        }
        return false
}

// validateType guarantees that the value is valid and assignable to the type.
func (s *state) validateType(value reflect.Value, typ reflect.Type) reflect.Value {
        if !value.IsValid() {
                if typ == nil {
                        // An untyped nil interface{}. Accept as a proper nil value.
                        return reflect.ValueOf(nil)
                }
                if canBeNil(typ) {
                        // Like above, but use the zero value of the non-nil type.
                        return reflect.Zero(typ)
                }
                s.errorf("invalid value; expected %s", typ)
        }
        if typ == reflectValueType && value.Type() != typ {
                return reflect.ValueOf(value)
        }
        if typ != nil && !value.Type().AssignableTo(typ) {
                if value.Kind() == reflect.Interface && !value.IsNil() {
                        value = value.Elem()
                        if value.Type().AssignableTo(typ) {
                                return value
                        }
                        // fallthrough
                }
                // Does one dereference or indirection work? We could do more, as we
                // do with method receivers, but that gets messy and method receivers
                // are much more constrained, so it makes more sense there than here.
                // Besides, one is almost always all you need.
                switch {
                case value.Kind() == reflect.Pointer && value.Type().Elem().AssignableTo(typ):
                        value = value.Elem()
                        if !value.IsValid() {
                                s.errorf("dereference of nil pointer of type %s", typ)
                        }
                case reflect.PointerTo(value.Type()).AssignableTo(typ) && value.CanAddr():
                        value = value.Addr()
                default:
                        s.errorf("wrong type for value; expected %s; got %s", typ, value.Type())
                }
        }
        return value
}

func (s *state) evalArg(dot reflect.Value, typ reflect.Type, n parse.Node) reflect.Value {
        s.at(n)
        switch arg := n.(type) {
        case *parse.DotNode:
                return s.validateType(dot, typ)
        case *parse.NilNode:
                if canBeNil(typ) {
                        return reflect.Zero(typ)
                }
                s.errorf("cannot assign nil to %s", typ)
        case *parse.FieldNode:
                return s.validateType(s.evalFieldNode(dot, arg, []parse.Node{n}, missingVal), typ)
        case *parse.VariableNode:
                return s.validateType(s.evalVariableNode(dot, arg, nil, missingVal), typ)
        case *parse.PipeNode:
                return s.validateType(s.evalPipeline(dot, arg), typ)
        case *parse.IdentifierNode:
                return s.validateType(s.evalFunction(dot, arg, arg, nil, missingVal), typ)
        case *parse.ChainNode:
                return s.validateType(s.evalChainNode(dot, arg, nil, missingVal), typ)
        }
        switch typ.Kind() {
        case reflect.Bool:
                return s.evalBool(typ, n)
        case reflect.Complex64, reflect.Complex128:
                return s.evalComplex(typ, n)
        case reflect.Float32, reflect.Float64:
                return s.evalFloat(typ, n)
        case reflect.Int, reflect.Int8, reflect.Int16, reflect.Int32, reflect.Int64:
                return s.evalInteger(typ, n)
        case reflect.Interface:
                if typ.NumMethod() == 0 {
                        return s.evalEmptyInterface(dot, n)
                }
        case reflect.Struct:
                if typ == reflectValueType {
                        return reflect.ValueOf(s.evalEmptyInterface(dot, n))
                }
        case reflect.String:
                return s.evalString(typ, n)
        case reflect.Uint, reflect.Uint8, reflect.Uint16, reflect.Uint32, reflect.Uint64, reflect.Uintptr:
                return s.evalUnsignedInteger(typ, n)
        }
        s.errorf("can't handle %s for arg of type %s", n, typ)
        panic("not reached")
}

func (s *state) evalBool(typ reflect.Type, n parse.Node) reflect.Value {
        s.at(n)
        if n, ok := n.(*parse.BoolNode); ok {
                value := reflect.New(typ).Elem()
                value.SetBool(n.True)
                return value
        }
        s.errorf("expected bool; found %s", n)
        panic("not reached")
}

func (s *state) evalString(typ reflect.Type, n parse.Node) reflect.Value {
        s.at(n)
        if n, ok := n.(*parse.StringNode); ok {
                value := reflect.New(typ).Elem()
                value.SetString(n.Text)
                return value
        }
        s.errorf("expected string; found %s", n)
        panic("not reached")
}

func (s *state) evalInteger(typ reflect.Type, n parse.Node) reflect.Value {
        s.at(n)
        if n, ok := n.(*parse.NumberNode); ok && n.IsInt {
                value := reflect.New(typ).Elem()
                value.SetInt(n.Int64)
                return value
        }
        s.errorf("expected integer; found %s", n)
        panic("not reached")
}

func (s *state) evalUnsignedInteger(typ reflect.Type, n parse.Node) reflect.Value {
        s.at(n)
        if n, ok := n.(*parse.NumberNode); ok && n.IsUint {
                value := reflect.New(typ).Elem()
                value.SetUint(n.Uint64)
                return value
        }
        s.errorf("expected unsigned integer; found %s", n)
        panic("not reached")
}

func (s *state) evalFloat(typ reflect.Type, n parse.Node) reflect.Value {
        s.at(n)
        if n, ok := n.(*parse.NumberNode); ok && n.IsFloat {
                value := reflect.New(typ).Elem()
                value.SetFloat(n.Float64)
                return value
        }
        s.errorf("expected float; found %s", n)
        panic("not reached")
}

func (s *state) evalComplex(typ reflect.Type, n parse.Node) reflect.Value {
        if n, ok := n.(*parse.NumberNode); ok && n.IsComplex {
                value := reflect.New(typ).Elem()
                value.SetComplex(n.Complex128)
                return value
        }
        s.errorf("expected complex; found %s", n)
        panic("not reached")
}

func (s *state) evalEmptyInterface(dot reflect.Value, n parse.Node) reflect.Value {
        s.at(n)
        switch n := n.(type) {
        case *parse.BoolNode:
                return reflect.ValueOf(n.True)
        case *parse.DotNode:
                return dot
        case *parse.FieldNode:
                return s.evalFieldNode(dot, n, nil, missingVal)
        case *parse.IdentifierNode:
                return s.evalFunction(dot, n, n, nil, missingVal)
        case *parse.NilNode:
                // NilNode is handled in evalArg, the only place that calls here.
                s.errorf("evalEmptyInterface: nil (can't happen)")
        case *parse.NumberNode:
                return s.idealConstant(n)
        case *parse.StringNode:
                return reflect.ValueOf(n.Text)
        case *parse.VariableNode:
                return s.evalVariableNode(dot, n, nil, missingVal)
        case *parse.PipeNode:
                return s.evalPipeline(dot, n)
        }
        s.errorf("can't handle assignment of %s to empty interface argument", n)
        panic("not reached")
}

// indirect returns the item at the end of indirection, and a bool to indicate
// if it's nil. If the returned bool is true, the returned value's kind will be
// either a pointer or interface.
func indirect(v reflect.Value) (rv reflect.Value, isNil bool) {
        for ; v.Kind() == reflect.Pointer || v.Kind() == reflect.Interface; v = v.Elem() {
                if v.IsNil() {
                        return v, true
                }
        }
        return v, false
}

// indirectInterface returns the concrete value in an interface value,
// or else the zero reflect.Value.
// That is, if v represents the interface value x, the result is the same as reflect.ValueOf(x):
// the fact that x was an interface value is forgotten.
func indirectInterface(v reflect.Value) reflect.Value {
        if v.Kind() != reflect.Interface {
                return v
        }
        if v.IsNil() {
                return reflect.Value{}
        }
        return v.Elem()
}

// printValue writes the textual representation of the value to the output of
// the template.
func (s *state) printValue(n parse.Node, v reflect.Value) {
        s.at(n)
        iface, ok := printableValue(v)
        if !ok {
                s.errorf("can't print %s of type %s", n, v.Type())
        }
        _, err := fmt.Fprint(s.wr, iface)
        if err != nil {
                s.writeError(err)
        }
}

// printableValue returns the, possibly indirected, interface value inside v that
// is best for a call to formatted printer.
func printableValue(v reflect.Value) (any, bool) {
        if v.Kind() == reflect.Pointer {
                v, _ = indirect(v) // fmt.Fprint handles nil.
        }
        if !v.IsValid() {
                return "<no value>", true
        }

        if !v.Type().Implements(errorType) && !v.Type().Implements(fmtStringerType) {
                if v.CanAddr() && (reflect.PointerTo(v.Type()).Implements(errorType) || reflect.PointerTo(v.Type()).Implements(fmtStringerType)) {
                        v = v.Addr()
                } else {
                        switch v.Kind() {
                        case reflect.Chan, reflect.Func:
                                return nil, false
                        }
                }
        }
        return v.Interface(), true
}