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[2603:7000:9500:34a5::1db4]) by smtp.gmail.com with ESMTPSA id g3-20020ac84803000000b003be56bdd3b1sm8817221qtq.92.2023.03.01.12.34.03 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 01 Mar 2023 12:34:04 -0800 (PST) Date: Wed, 1 Mar 2023 15:34:02 -0500 From: Marek Polacek To: Jason Merrill Cc: GCC Patches Subject: Re: [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: 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: Ping. On Tue, Feb 07, 2023 at 11:46:10AM -0500, Marek Polacek wrote: > 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 > Marek