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[2603:7000:9500:34a5::1db4]) by smtp.gmail.com with ESMTPSA id l11-20020a05620a28cb00b007259807a512sm10115873qkp.12.2023.02.07.08.46.11 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 07 Feb 2023 08:46:12 -0800 (PST) Date: Tue, 7 Feb 2023 11:46:10 -0500 From: Marek Polacek To: Jason Merrill Cc: GCC Patches Subject: [PATCH v4] c++: -Wdangling-reference with reference wrapper [PR107532] Message-ID: References: <20230118175200.365397-1-polacek@redhat.com> <0024ebdb-ecf2-d841-0e73-7063e4dcb9fe@redhat.com> <61a408eb-d9a8-6c3b-4d7a-367fe0d3997e@redhat.com> MIME-Version: 1.0 In-Reply-To: <61a408eb-d9a8-6c3b-4d7a-367fe0d3997e@redhat.com> User-Agent: Mutt/2.2.9 (2022-11-12) X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=us-ascii Content-Disposition: inline X-Spam-Status: No, score=-12.7 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,RCVD_IN_DNSWL_NONE,RCVD_IN_MSPIKE_H2,SPF_HELO_NONE,SPF_NONE,TXREP autolearn=ham autolearn_force=no version=3.4.6 X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on server2.sourceware.org List-Id: On Sun, Feb 05, 2023 at 05:25:25PM -0800, Jason Merrill wrote: > 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. > > > > > > > > > > > > > > > > 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. > > > > > > > > > > > > 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. > > > > > > > > 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::inner (&TARGET_EXPR ) > > > > or > > > > Ref::inner (&TARGET_EXPR )>) > > > > 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. Ah yes, I can do that if I introduce a parameter that tells us if we're processing an argument or not. I think I'm finally more or less satisfied with the patch, 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. 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 | 97 +++++++++++++++---- .../g++.dg/warn/Wdangling-reference8.C | 77 +++++++++++++++ .../g++.dg/warn/Wdangling-reference9.C | 21 ++++ 3 files changed, 178 insertions(+), 17 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 f7c5d9da94b..2a8edc2e7e2 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 @@ -13791,12 +13830,39 @@ 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 (TREE_CODE (expr) == ADDR_EXPR) + expr = TREE_OPERAND (expr, 0); + + if (arg_p && expr_represents_temporary_p (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. */ + tree e = expr; + while (handled_component_p (e)) + e = TREE_OPERAND (e, 0); + e = TREE_TYPE (e); + if (!CLASS_TYPE_P (e) || !reference_like_class_p (e)) + return expr; + } + switch (TREE_CODE (expr)) { case CALL_EXPR: @@ -13829,7 +13895,8 @@ 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 (!arg_p + && (!(TYPE_REF_OBJ_P (rettype) || std_pair_ref_ref_p (rettype)))) return NULL_TREE; /* Here we're looking to see if any of the arguments is a temporary @@ -13842,14 +13909,10 @@ 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(...); @@ -13866,15 +13929,15 @@ 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; } @@ -13917,7 +13980,7 @@ 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; +} base-commit: f661c0bb6371f355966a67b5ce71398e80792948 -- 2.39.1