| Message ID | Y8n2ayXMZf+dYsqi@redhat.com |
|---|---|
| State | New |
| Headers | show |
| Series | [v3] c++: -Wdangling-reference with reference wrapper [PR107532] | expand |
On 1/19/23 21:03, Marek Polacek wrote: > 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))) Ah, in this case I was thinking rather than return we would want to look through to the initializer of the reference wrapper, and warn if that's a temporary, so we can catch the *2 cases in your tests. So, treating ref-like classes as much like references as we can. Some of your v1 patch ought to be useful in implementing this, but only looking through one call at a time, not all of them like that patch. > + 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
On Fri, Jan 20, 2023 at 03:19:54PM -0500, Jason Merrill wrote: > On 1/19/23 21:03, Marek Polacek wrote: > > 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))) > > Ah, in this case I was thinking rather than return we would want to look > through to the initializer of the reference wrapper, and warn if that's a > temporary, so we can catch the *2 cases in your tests. > > So, treating ref-like classes as much like references as we can. Some of > your v1 patch ought to be useful in implementing this, but only looking > through one call at a time, not all of them like that patch. Maybe this one, then? I still have to loop through the calls though; EXPR in do_warn_dangling_reference can be e.g. Ref<const Plane>::inner (&TARGET_EXPR <D.2839, FrameMetadata::planes ((const struct FrameMetadata *) fm)>) or Ref<const Plane>::inner (&TARGET_EXPR <D.2908, FrameMetadata::planes (&TARGET_EXPR <D.2898, {.p_={.bytesused=0}}>)>) and we want to warn only about the latter, but that means that I need to look into the nested call 'planes' to see if the initializer was a temporary. With this, we warn for the *2 cases too. 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 | 70 ++++++++++++++++- .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++ .../g++.dg/warn/Wdangling-reference9.C | 21 +++++ 3 files changed, 167 insertions(+), 1 deletion(-) 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 5715a7cd1de..137870670e7 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 @@ -13850,7 +13889,36 @@ do_warn_dangling_reference (tree expr) if (TREE_CODE (arg) == ADDR_EXPR) arg = TREE_OPERAND (arg, 0); if (expr_represents_temporary_p (arg)) - return expr; + { + /* 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) + && i == 0 + && reference_like_class_p (CP_DECL_CONTEXT (fndecl))) + { + /* Let's see what the class object was initialized with. + ARG is the TARGET_EXPR initializer; it may look like + TARGET_EXPR <D.2839, A::foo (decl)> + or + TARGET_EXPR <D.2839, A::foo (&TARGET_EXPR <...>)> + We should only warn for the second case. */ + while (handled_component_p (arg)) + arg = TREE_OPERAND (arg, 0); + arg = TARGET_EXPR_INITIAL (arg); + /* Walk the call chain to the original object and see if + it was a temporary. */ + while (TREE_CODE (arg) == CALL_EXPR) + arg = tree_strip_nop_conversions (CALL_EXPR_ARG (arg, 0)); + if (TREE_CODE (arg) == ADDR_EXPR) + arg = TREE_OPERAND (arg, 0); + if (!expr_represents_temporary_p (arg)) + break; + } + return expr; + } /* Don't warn about member function like: std::any a(...); S& s = a.emplace<S>({0}, 0); 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 <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(); // { 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<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(); // { 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<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..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 <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(); // { dg-warning "dangling reference" } + (void) a2; +} base-commit: 327d45c57ebd2655a7599df0f01b8b5e2f82eda7
On 1/24/23 17:49, Marek Polacek wrote: > On Fri, Jan 20, 2023 at 03:19:54PM -0500, Jason Merrill wrote: >> On 1/19/23 21:03, Marek Polacek wrote: >>> 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))) >> >> Ah, in this case I was thinking rather than return we would want to look >> through to the initializer of the reference wrapper, and warn if that's a >> temporary, so we can catch the *2 cases in your tests. >> >> So, treating ref-like classes as much like references as we can. Some of >> your v1 patch ought to be useful in implementing this, but only looking >> through one call at a time, not all of them like that patch. > > Maybe this one, then? I still have to loop through the calls though; EXPR in > do_warn_dangling_reference can be e.g. > > Ref<const Plane>::inner (&TARGET_EXPR <D.2839, FrameMetadata::planes ((const struct FrameMetadata *) fm)>) > > or > > Ref<const Plane>::inner (&TARGET_EXPR <D.2908, FrameMetadata::planes (&TARGET_EXPR <D.2898, {.p_={.bytesused=0}}>)>) > > and we want to warn only about the latter, but that means that I need to > look into the nested call 'planes' to see if the initializer was a temporary. Right, but I was thinking we want to recurse like a few lines above, rather than loop. > With this, we warn for the *2 cases too. > > 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 | 70 ++++++++++++++++- > .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++++++ > .../g++.dg/warn/Wdangling-reference9.C | 21 +++++ > 3 files changed, 167 insertions(+), 1 deletion(-) > 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 5715a7cd1de..137870670e7 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 > @@ -13850,7 +13889,36 @@ do_warn_dangling_reference (tree expr) > if (TREE_CODE (arg) == ADDR_EXPR) > arg = TREE_OPERAND (arg, 0); > if (expr_represents_temporary_p (arg)) > - return expr; > + { > + /* 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) > + && i == 0 > + && reference_like_class_p (CP_DECL_CONTEXT (fndecl))) > + { > + /* Let's see what the class object was initialized with. > + ARG is the TARGET_EXPR initializer; it may look like > + TARGET_EXPR <D.2839, A::foo (decl)> > + or > + TARGET_EXPR <D.2839, A::foo (&TARGET_EXPR <...>)> > + We should only warn for the second case. */ > + while (handled_component_p (arg)) > + arg = TREE_OPERAND (arg, 0); > + arg = TARGET_EXPR_INITIAL (arg); > + /* Walk the call chain to the original object and see if > + it was a temporary. */ > + while (TREE_CODE (arg) == CALL_EXPR) > + arg = tree_strip_nop_conversions (CALL_EXPR_ARG (arg, 0)); > + if (TREE_CODE (arg) == ADDR_EXPR) > + arg = TREE_OPERAND (arg, 0); > + if (!expr_represents_temporary_p (arg)) > + break; > + } > + return expr; > + } > /* Don't warn about member function like: > std::any a(...); > S& s = a.emplace<S>({0}, 0); > 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 <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(); // { 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<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(); // { 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<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..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 <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(); // { dg-warning "dangling reference" } > + (void) a2; > +} > > base-commit: 327d45c57ebd2655a7599df0f01b8b5e2f82eda7
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; +}