From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) by sourceware.org (Postfix) with ESMTPS id 3BB853858D28 for ; Sat, 4 Mar 2023 02:30:45 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.4.2 sourceware.org 3BB853858D28 Authentication-Results: sourceware.org; dmarc=pass (p=none dis=none) header.from=redhat.com Authentication-Results: sourceware.org; spf=pass smtp.mailfrom=redhat.com DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1677897044; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=Ateb68jYX7+Gzh+KPQDjUevAZ6bDEGjYUBBqmLysb7A=; b=B3eMxCs82XuhIWjEOyKEkdlQVCx6B3/fUTFoR2z94FoFZRWJt/5FdPZh9YcFWN44GJ/YYq tRxC59k6lhiTG5o45xv0JraxWoiNvSydfjcTUAMSulmojkxGgHBTib+jPzDfNIGF0BiQrL Y37NNy9+wkmDnh5hcmUiPWeW/tvN5Xo= Received: from mail-qk1-f200.google.com (mail-qk1-f200.google.com [209.85.222.200]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-249-2agzIpU7Njm4ozfZGzwCIw-1; Fri, 03 Mar 2023 21:30:43 -0500 X-MC-Unique: 2agzIpU7Njm4ozfZGzwCIw-1 Received: by mail-qk1-f200.google.com with SMTP id ou5-20020a05620a620500b007423e532628so2470898qkn.5 for ; Fri, 03 Mar 2023 18:30:43 -0800 (PST) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20210112; t=1677897042; h=content-transfer-encoding:in-reply-to:from:references:cc:to :content-language:subject:user-agent:mime-version:date:message-id :x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=Ateb68jYX7+Gzh+KPQDjUevAZ6bDEGjYUBBqmLysb7A=; b=TLea6geTgCzAKWj2YpXJ5ydEQC++YML0dxkQpVwcnPN5QJ8+818jS4u74OvtHeN//6 YqOZdNr1XF0IQHa9Ls1iKDfGHtWBnp4gVPb0YKviqj/6r4OifiSxGC3j17PvsjHvjnRC +ysSKym3hsvlF93dQU3wERUoHfjzwvKiygMuXnP4a2PKexoO/jONicfpYTUgXcnDfhBm ZCRCBUUiSvRJXaBl46uxPohVYEtVHVJx8lfw17AOhPSGjpZoRRkjQPoIbe2JhCDkp9OD bt3I7zuCBroEPVBKZHi4TkRm80/WoA5yGU36CQ8fj2R0zEocHCkb24BWcTLB9lE/LX3f K6Xg== X-Gm-Message-State: AO0yUKX7THzmHQawH4I86PHotwsTIxVDyFyVeeCg5Ab2XuPdxNzGcmCp SL55I9PZH2WKsYSjCf/Cgvifh3a4o4LFoS87cobbGx/070KuYGPAgcBb6ODIESfsYTleZbG1SXr cGkwHU+T6QMhpc/QpQw== X-Received: by 2002:ac8:5bc9:0:b0:3bf:cd81:3a31 with SMTP id b9-20020ac85bc9000000b003bfcd813a31mr7222667qtb.65.1677897042538; Fri, 03 Mar 2023 18:30:42 -0800 (PST) X-Google-Smtp-Source: AK7set/zkmbYLKdXazNOUiK5qpY2OL5igE/Wd7SLRImj0zstPw8p9OUIm3oAzXe1EKFE9QhMV5BLYg== X-Received: by 2002:ac8:5bc9:0:b0:3bf:cd81:3a31 with SMTP id b9-20020ac85bc9000000b003bfcd813a31mr7222629qtb.65.1677897041953; Fri, 03 Mar 2023 18:30:41 -0800 (PST) Received: from [192.168.1.108] (130-44-159-43.s15913.c3-0.arl-cbr1.sbo-arl.ma.cable.rcncustomer.com. [130.44.159.43]) by smtp.gmail.com with ESMTPSA id d64-20020a37b443000000b0073b587194d0sm2843153qkf.104.2023.03.03.18.30.40 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Fri, 03 Mar 2023 18:30:41 -0800 (PST) Message-ID: <14932057-e72d-3d8d-eed1-1bd2a949742d@redhat.com> Date: Fri, 3 Mar 2023 21:30:38 -0500 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:102.0) Gecko/20100101 Thunderbird/102.8.0 Subject: Re: [PATCH v5] c++: -Wdangling-reference with reference wrapper [PR107532] To: Marek Polacek Cc: GCC Patches References: <0024ebdb-ecf2-d841-0e73-7063e4dcb9fe@redhat.com> <61a408eb-d9a8-6c3b-4d7a-367fe0d3997e@redhat.com> <81d1c004-82c2-096f-d2f0-48e365dd954a@redhat.com> From: Jason Merrill In-Reply-To: X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Language: en-US Content-Type: text/plain; charset=UTF-8; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-12.6 required=5.0 tests=BAYES_00,DKIMWL_WL_HIGH,DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF,GIT_PATCH_0,NICE_REPLY_A,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 3/3/23 12:50, Marek Polacek wrote: > On Fri, Mar 03, 2023 at 11:25:06AM -0500, Jason Merrill wrote: >> On 3/2/23 16:24, Marek Polacek wrote: >>> On Wed, Mar 01, 2023 at 04:53:23PM -0500, Jason Merrill wrote: >>>>> @@ -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); >>>> >>>> I think if we move this here, we also need to check that expr before >>>> STRIP_NOPS had REFERENCE_TYPE. OK with that change. >>> >>> Sorry but I don't think I can do that. There can be CONVERT_EXPRs >>> that need to be stripped, whether arg_p or !arg_p. For example, we can get >>> (const int *) f ((const int &) &TARGET_EXPR >) >>> for >>> const int& r5 = (42, f(10)); >> >> I meant that we only want to strip ADDR_EXPR if 'expr' at the start of the >> function had REFERENCE_TYPE, corresponding to >> >>> /* Check that this argument initializes a reference, except >>> for >>> the argument initializing the object of a member function. */ >>> if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl) >>> && !TYPE_REF_P (TREE_TYPE (arg))) >>> continue; >> >> above the code for stripping an ADDR_EXPR from an argument that your patch >> removes. > > I see. > >> If the original expr is a pointer rather than a reference, we don't want to >> complain about it pointing to a temporary. > > Ug, I can't make it work. When we recurse, I can no longer check > fndecl. How about just moving the stripping back where it was? Sure. > -- >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 | 92 ++++++++++++++++--- > .../g++.dg/warn/Wdangling-reference8.C | 77 ++++++++++++++++ > .../g++.dg/warn/Wdangling-reference9.C | 21 +++++ > 3 files changed, 176 insertions(+), 14 deletions(-) > create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference8.C > create mode 100644 gcc/testsuite/g++.dg/warn/Wdangling-reference9.C > > diff --git a/gcc/cp/call.cc b/gcc/cp/call.cc > index 048b2b052f8..62536573633 100644 > --- a/gcc/cp/call.cc > +++ b/gcc/cp/call.cc > @@ -13779,6 +13779,45 @@ 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 > @@ -13793,12 +13832,37 @@ std_pair_ref_ref_p (tree t) > const int& y = (f(1), 42); // NULL_TREE > const int& z = f(f(1)); // f(f(1)) > > - EXPR is the initializer. */ > + EXPR is the initializer. If ARG_P is true, we're processing an argument > + to a function; the point is to distinguish between, for example, > + > + Ref::inner (&TARGET_EXPR ) > + > + where we shouldn't warn, and > + > + Ref::inner (&TARGET_EXPR )>) > + > + where we should warn (Ref is a reference_like_class_p so we see through > + it. */ > > static tree > -do_warn_dangling_reference (tree expr) > +do_warn_dangling_reference (tree expr, bool arg_p) > { > STRIP_NOPS (expr); > + > + if (arg_p && expr_represents_temporary_p (expr)) > + { > + /* An attempt to reduce the number of -Wdangling-reference > + false positives concerning reference wrappers (c++/107532). > + 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: > @@ -13831,7 +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)))) Instead of checking !arg_p maybe the std_pair_ref_ref_p call should change to reference_like_class_p (which in turn should check std_pair_ref_ref_p)? > return NULL_TREE; > > /* Here we're looking to see if any of the arguments is a temporary > @@ -13844,14 +13909,13 @@ do_warn_dangling_reference (tree expr) > if (!DECL_NONSTATIC_MEMBER_FUNCTION_P (fndecl) > && !TYPE_REF_P (TREE_TYPE (arg))) > continue; > - /* It could also be another call taking a temporary and returning > - it and initializing this reference parameter. */ > - if (do_warn_dangling_reference (arg)) > - return expr; > STRIP_NOPS (arg); > if (TREE_CODE (arg) == ADDR_EXPR) > arg = TREE_OPERAND (arg, 0); > - if (expr_represents_temporary_p (arg)) > + /* Recurse to see if the argument is a temporary. It could also > + be another call taking a temporary and returning it and > + initializing this reference parameter. */ > + if (do_warn_dangling_reference (arg, /*arg_p=*/true)) > return expr; > /* Don't warn about member function like: > std::any a(...); > @@ -13868,15 +13932,15 @@ do_warn_dangling_reference (tree expr) > return NULL_TREE; > } > case COMPOUND_EXPR: > - return do_warn_dangling_reference (TREE_OPERAND (expr, 1)); > + return do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p); > case COND_EXPR: > - if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1))) > + if (tree t = do_warn_dangling_reference (TREE_OPERAND (expr, 1), arg_p)) > return t; > - return do_warn_dangling_reference (TREE_OPERAND (expr, 2)); > + return do_warn_dangling_reference (TREE_OPERAND (expr, 2), arg_p); > case PAREN_EXPR: > - return do_warn_dangling_reference (TREE_OPERAND (expr, 0)); > + return do_warn_dangling_reference (TREE_OPERAND (expr, 0), arg_p); > case TARGET_EXPR: > - return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr)); > + return do_warn_dangling_reference (TARGET_EXPR_INITIAL (expr), arg_p); > default: > return NULL_TREE; > } > @@ -13919,7 +13983,7 @@ 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: ce1c99f1ccd7b1229a4f8531d6b6de6cf571a9ef