From 80f0052b3924569af904d1bab0858fe985f33a94 Mon Sep 17 00:00:00 2001 From: Marek Polacek Date: Tue, 17 Jan 2023 17:34:58 -0500 Subject: [PATCH] c++: -Wdangling-reference with reference wrapper [PR107532] Here, -Wdangling-reference triggers where it probably shouldn't, causing some grief. The code in question uses a reference wrapper with a member function returning a reference to a subobject of a non-temporary object: const Plane & meta = fm.planes().inner(); I've tried a few approaches, e.g., checking that the member function's return type is the same as the type of the enclosing class (which is the case for member functions returning *this), but that then breaks Wdangling-reference4.C with std::optional. This patch adjusts do_warn_dangling_reference so that we look through reference wrapper classes (meaning, has a reference member and a constructor taking the same reference type, or is std::reference_wrapper or std::ranges::ref_view) and don't warn for them, supposing that the member function returns a reference to a non-temporary object. PR c++/107532 gcc/cp/ChangeLog: * call.cc (reference_like_class_p): New. (do_warn_dangling_reference): Add new bool parameter. See through reference_like_class_p. gcc/testsuite/ChangeLog: * g++.dg/warn/Wdangling-reference8.C: New test. * g++.dg/warn/Wdangling-reference9.C: New test. --- gcc/cp/call.cc | 98 ++++++++++++++++++++---- gcc/testsuite/g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++ gcc/testsuite/g++.dg/warn/Wdangling-reference9.C | 21 +++++ 3 files changed, 182 insertions(+), 14 deletions(-) create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc index 048b2b052f8..3dfa12a0733 100644 --- a/gcc/cp/call.cc +++ b/gcc/cp/call.cc @@ -13779,6 +13779,52 @@ std_pair_ref_ref_p (tree t) return true; } +/* Return true if a class CTYPE is either std::reference_wrapper or + std::ref_view, or a reference wrapper class. We consider a class + a reference wrapper class if it has a reference member and a + constructor taking the same reference type. */ + +static bool +reference_like_class_p (tree ctype) +{ + if (!CLASS_TYPE_P (ctype)) + return false; + + /* Also accept a std::pair. */ + if (std_pair_ref_ref_p (ctype)) + return true; + + tree tdecl = TYPE_NAME (TYPE_MAIN_VARIANT (ctype)); + if (decl_in_std_namespace_p (tdecl)) + { + tree name = DECL_NAME (tdecl); + return (name + && (id_equal (name, "reference_wrapper") + || id_equal (name, "ref_view"))); + } + for (tree fields = TYPE_FIELDS (ctype); + fields; + fields = DECL_CHAIN (fields)) + { + if (TREE_CODE (fields) != FIELD_DECL || DECL_ARTIFICIAL (fields)) + continue; + tree type = TREE_TYPE (fields); + if (!TYPE_REF_P (type)) + continue; + /* OK, the field is a reference member. Do we have a constructor + taking its type? */ + for (tree fn : ovl_range (CLASSTYPE_CONSTRUCTORS (ctype))) + { + tree args = FUNCTION_FIRST_USER_PARMTYPE (fn); + if (args + && same_type_p (TREE_VALUE (args), type) + && TREE_CHAIN (args) == void_list_node) + return true; + } + } + return false; +} + /* Helper for maybe_warn_dangling_reference to find a problematic CALL_EXPR that initializes the LHS (and at least one of its arguments represents a temporary, as outlined in maybe_warn_dangling_reference), or NULL_TREE @@ -13793,12 +13839,37 @@ std_pair_ref_ref_p (tree t) const int& y = (f(1), 42); // NULL_TREE const int& z = f(f(1)); // f(f(1)) - EXPR is the initializer. */ + EXPR is the initializer. If ARG_P is true, we're processing an argument + to a function; the point is to distinguish between, for example, + + Ref::inner (&TARGET_EXPR ) + + where we shouldn't warn, and + + Ref::inner (&TARGET_EXPR )>) + + where we should warn (Ref is a reference_like_class_p so we see through + it. */ static tree -do_warn_dangling_reference (tree expr) +do_warn_dangling_reference (tree expr, bool arg_p) { STRIP_NOPS (expr); + + if (arg_p && expr_represents_temporary_p (expr)) + { + /* An attempt to reduce the number of -Wdangling-reference + false positives concerning reference wrappers (c++/107532). + When we encounter a reference_like_class_p, we don't warn + just yet; instead, we keep recursing to see if there were + any temporaries behind the reference-wrapper class. */ + tree e = expr; + while (handled_component_p (e)) + e = TREE_OPERAND (e, 0); + if (!reference_like_class_p (TREE_TYPE (e))) + return expr; + } + switch (TREE_CODE (expr)) { case CALL_EXPR: @@ -13831,7 +13902,7 @@ do_warn_dangling_reference (tree expr) std::pair v = std::minmax(1, 2); which also creates a dangling reference, because std::minmax returns std::pair(b, a). */ - if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype))) + if (!(TYPE_REF_OBJ_P (rettype) || reference_like_class_p (rettype))) return NULL_TREE; /* Here we're looking to see if any of the arguments is a temporary @@ -13844,14 +13915,13 @@ do_warn_dangling_reference (tree expr) if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl) && !TYPE_REF_P (TREE_TYPE (arg))) continue; - /* It could also be another call taking a temporary and returning - it and initializing this reference parameter. */ - if (do_warn_dangling_reference (arg)) - return expr; STRIP_NOPS (arg); if (TREE_CODE (arg) == ADDR_EXPR) arg = TREE_OPERAND (arg, 0); - if (expr_represents_temporary_p (arg)) + /* Recurse to see if the argument is a temporary. It could also + be another call taking a temporary and returning it and + initializing this reference parameter. */ + if (do_warn_dangling_reference (arg, /*arg_p=*/true)) return expr; /* Don't warn about member function like: std::any a(...); @@ -13868,15 +13938,15 @@ do_warn_dangling_reference (tree expr) return NULL_TREE; } case COMPOUND_EXPR: - return do_warn_dangling_reference (TREE_OPERAND (expr, 1)); + return do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p); case COND_EXPR: - if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1))) + if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p)) return t; - return do_warn_dangling_reference (TREE_OPERAND (expr, 2)); + return do_warn_dangling_reference (TREE_OPERAND (expr, 2), arg_p); case PAREN_EXPR: - return do_warn_dangling_reference (TREE_OPERAND (expr, 0)); + return do_warn_dangling_reference (TREE_OPERAND (expr, 0), arg_p); case TARGET_EXPR: - return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr)); + return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr), arg_p); default: return NULL_TREE; } @@ -13919,7 +13989,7 @@ maybe_warn_dangling_reference (const_tree decl, tree init) = make_temp_override (global_dc->dc_warn_system_headers, (!in_system_header_at (DECL_SOURCE_LOCATION (decl)) || global_dc->dc_warn_system_headers)); - if (tree call = do_warn_dangling_reference (init)) + if (tree call = do_warn_dangling_reference (init, /*arg_p=*/false)) { auto_diagnostic_group d; if (warning_at (DECL_SOURCE_LOCATION (decl), OPT_Wdangling_reference, diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C new file mode 100644 index 00000000000..330de1fd05d --- /dev/null +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference8.C @@ -0,0 +1,77 @@ +// PR c++/107532 +// { dg-do compile { target c++11 } } +// { dg-options "-Wdangling-reference" } + +struct Plane { unsigned int bytesused; }; + +// Passes a reference through. Does not change lifetime. +template +struct Ref { + const T& i_; + Ref(const T & i) : i_(i) {} + const T & inner(); +}; + +struct FrameMetadata { + Ref planes() const { return p_; } + + Plane p_; +}; + +void bar(const Plane & meta); +void foo(const FrameMetadata & fm) +{ + const Plane & meta = fm.planes().inner(); + bar(meta); + const Plane & meta2 = FrameMetadata().planes().inner(); // { dg-warning "dangling reference" } + bar(meta2); +} + +struct S { + const S& self () { return *this; } +} s; + +const S& r1 = s.self(); +const S& r2 = S().self(); // { dg-warning "dangling reference" } + +struct D { +}; + +struct C { + D d; + Ref get() const { return d; } +}; + +struct B { + C c; + const C& get() const { return c; } + B(); +}; + +struct A { + B b; + const B& get() const { return b; } +}; + +void +g (const A& a) +{ + const auto& d1 = a.get().get().get().inner(); + (void) d1; + const auto& d2 = A().get().get().get().inner(); // { dg-warning "dangling reference" } + (void) d2; + const auto& d3 = A().b.get().get().inner(); // { dg-warning "dangling reference" } + (void) d3; + const auto& d4 = a.b.get().get().inner(); + (void) d4; + const auto& d5 = a.b.c.get().inner(); + (void) d5; + const auto& d6 = A().b.c.get().inner(); // { dg-warning "dangling reference" } + (void) d6; + Plane p; + Ref r(p); + const auto& d7 = r.inner(); + (void) d7; + const auto& d8 = Ref(p).inner(); + (void) d8; +} diff --git a/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C new file mode 100644 index 00000000000..9ad83f7365e --- /dev/null +++ b/gcc/testsuite/g++.dg/warn/Wdangling-reference9.C @@ -0,0 +1,21 @@ +// PR c++/107532 +// { dg-do compile { target c++11 } } +// { dg-options "-Wdangling-reference" } + +#include + +struct X { int n; }; + +struct S { + std::reference_wrapper wrapit() const { return x; } + X x; +}; + +void +g (const S& s) +{ + const auto& a1 = s.wrapit().get(); + (void) a1; + const auto& a2 = S().wrapit().get(); // { dg-warning "dangling reference" } + (void) a2; +} -- 2.11.4.GIT