On Thu, Jan 19, 2023 at 01:02:02PM -0500, Jason Merrill wrote: > On 1/18/23 20:13, Marek Polacek wrote: > > On Wed, Jan 18, 2023 at 04:07:59PM -0500, Jason Merrill wrote: > > > On 1/18/23 12:52, Marek Polacek wrote: > > > > 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<std::string>. > > > > > > > > So I figured that perhaps we want to look at the object we're invoking > > > > the member function(s) on and see if that is a temporary, as in, don't > > > > warn about > > > > > > > > const Plane & meta = fm.planes().inner(); > > > > > > > > but do warn about > > > > > > > > const Plane & meta = FrameMetadata().planes().inner(); > > > > > > > > It's ugly, but better than asking users to add #pragmas into their code. > > > > > > Hmm, that doesn't seem right; the former is only OK because Ref is in > > > fact a > > > reference-like type. If planes() returned a class that held data, we > > > would > > > want to warn. > > > > Sure, it's always some kind of tradeoff with warnings :/. > > > In this case, we might recognize the reference-like class because it has a > > > reference member and a constructor taking the same reference type. > > > > That occurred to me too, but then I found out that std::reference_wrapper > > actually uses T*, not T&, as you say. But here's a patch to do that > > (I hope). > > > That wouldn't help with std::reference_wrapper or std::ref_view because > > > they > > > have pointer members instead of references, but perhaps loosening the > > > check > > > to include that case would make sense? > > > > Sorry, I don't understand what you mean by loosening the check. I could > > hardcode std::reference_wrapper and std::ref_view but I don't think that's > > what you meant. > > Indeed that's not what I meant, but as I was saying in our meeting I think > it's worth doing; the compiler has various tweaks to handle specific > standard-library classes better. Okay, done in the patch below. Except that I'm not including a test for std::ranges::ref_view because I don't really know how that works.
> > Surely I cannot _not_ warn for any class that contains a T*. > > I was thinking if a constructor takes a T& and the class has a T* that would > be close enough, though this also wouldn't handle the standard library > classes so the benefit is questionable. > > > Here's the patch so that we have some actual code to discuss... Thanks. > > > > -- >8 -- > > 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<std::string>. > > > > Perhaps we want to look at the member function's enclosing class > > to see if it's a reference wrapper class (meaning, has a reference > > member and a constructor taking the same reference type) and don't > > warn if so, supposing that the member function returns a reference > > to a non-temporary object. > > > > It's ugly, but better than asking users to add #pragmas into their code. > > > > PR c++/107532 > > > > gcc/cp/ChangeLog: > > > > * call.cc (do_warn_dangling_reference): Don't warn when the > > member function comes from a reference wrapper class. > > Let's factor the new code out into e.g. reference_like_class_p Done. Thanks, Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? -- >8 -- 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<std::string>. Perhaps we want to look at the member function's enclosing class to see if it's a reference wrapper class (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 if so, supposing that the member function returns a reference to a non-temporary object. It's ugly, but better than asking users to add #pragmas into their code. PR c++/107532 gcc/cp/ChangeLog: * call.cc (reference_like_class_p): New. (do_warn_dangling_reference): Don't warn when the member function comes from a 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 | 48 ++++++++++++ .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++ .../g++.dg/warn/Wdangling-reference9.C | 21 +++++ 3 files changed, 146 insertions(+) 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 991730713e6..672722998ee 100644 --- a/gcc/cp/call.cc +++ b/gcc/cp/call.cc @@ -13777,6 +13777,45 @@ 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) +{ + 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 @@ -13832,6 +13871,15 @@ do_warn_dangling_reference (tree expr) if (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype))) return NULL_TREE; + /* An attempt to reduce the number of -Wdangling-reference + false positives concerning reference wrappers (c++/107532). + Here we suppose that a member function of such a reference + wrapper class returns a reference to a non-temporary object. */ + if (DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl) + && !DECL_OVERLOADED_OPERATOR_P (fndecl) + && reference_like_class_p (CP_DECL_CONTEXT (fndecl))) + return NULL_TREE; + /* Here we're looking to see if any of the arguments is a temporary initializing a reference parameter. */ for (int i = 0; i < call_expr_nargs (expr); ++i) 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..4d585891fae --- /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 <typename T> +struct Ref { + const T& i_; + Ref(const T & i) : i_(i) {} + const T & inner(); +}; + +struct FrameMetadata { + Ref<const Plane> 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(); + 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<const D> 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(); + (void) d2; + const auto& d3 = A().b.get().get().inner(); + (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(); + (void) d6; + Plane p; + Ref<Plane> r(p); + const auto& d7 = r.inner(); + (void) d7; + const auto& d8 = Ref<Plane>(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..15c1f6b9dd2 --- /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 <functional> + +struct X { int n; }; + +struct S { + std::reference_wrapper<const X> 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(); + (void) a2; +} base-commit: 0846336de56119777861e02bf68f92a6af466000 -- 2.39.0