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[209.6.216.142]) by smtp.gmail.com with ESMTPSA id d203sm3252452qkc.10.2020.08.12.13.10.53 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 12 Aug 2020 13:10:53 -0700 (PDT) Subject: Re: [PATCH] c++: Fixing the wording of () aggregate-init [PR92812] To: Marek Polacek , GCC Patches References: <20200811233454.3074560-1-polacek@redhat.com> From: Jason Merrill Message-ID: <8410d28e-8052-c06f-cabf-9893dc20ae22@redhat.com> Date: Wed, 12 Aug 2020 16:10:52 -0400 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.6.0 MIME-Version: 1.0 In-Reply-To: <20200811233454.3074560-1-polacek@redhat.com> Content-Language: en-US X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: redhat.com Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit X-Spam-Status: No, score=-16.5 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_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: gcc-patches@gcc.gnu.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Gcc-patches mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Wed, 12 Aug 2020 20:10:59 -0000 On 8/11/20 7:34 PM, Marek Polacek wrote: > P1975R0 tweaks the static_cast wording: it says that "An expression e can be > explicitly converted to a type T if [...] T is an aggregate type having a first > element x and there is an implicit conversion sequence from e to the type of > x." This already works for classes, e.g.: > > struct Aggr { int x; int y; }; > Aggr a = static_cast(1); > > for which we create TARGET_EXPR . > > The proposal also mentions "If T is ``array of unknown bound of U'', > this direct-initialization defines the type of the expression as U[1]" which > suggest that this should work for arrays (they're aggregates too, after all): > > int (&&r)[3] = static_cast(42); > int (&&r2)[1] = static_cast(42); > > So I handled that specifically in build_static_cast_1: wrap the > expression in { } and initialize from that. For the 'r' case above > this creates TARGET_EXPR . > > There are multiple things in play, as usual, so the tests test brace > elision, narrowing, explicit constructors, and lifetime extension too. > I think it's in line with what we discussed on the core reflector. > > Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk? OK. > gcc/cp/ChangeLog: > > PR c++/92812 > * typeck.c (build_static_cast_1): Implement P1975R0 by allowing > static_cast to aggregate type. > > gcc/testsuite/ChangeLog: > > PR c++/92812 > * g++.dg/cpp2a/paren-init27.C: New test. > * g++.dg/cpp2a/paren-init28.C: New test. > * g++.dg/cpp2a/paren-init29.C: New test. > * g++.dg/cpp2a/paren-init30.C: New test. > * g++.dg/cpp2a/paren-init31.C: New test. > * g++.dg/cpp2a/paren-init32.C: New test. > --- > gcc/cp/typeck.c | 14 +++++++++++++ > gcc/testsuite/g++.dg/cpp2a/paren-init27.C | 24 +++++++++++++++++++++++ > gcc/testsuite/g++.dg/cpp2a/paren-init28.C | 15 ++++++++++++++ > gcc/testsuite/g++.dg/cpp2a/paren-init29.C | 15 ++++++++++++++ > gcc/testsuite/g++.dg/cpp2a/paren-init30.C | 23 ++++++++++++++++++++++ > gcc/testsuite/g++.dg/cpp2a/paren-init31.C | 10 ++++++++++ > gcc/testsuite/g++.dg/cpp2a/paren-init32.C | 21 ++++++++++++++++++++ > 7 files changed, 122 insertions(+) > create mode 100644 gcc/testsuite/g++.dg/cpp2a/paren-init27.C > create mode 100644 gcc/testsuite/g++.dg/cpp2a/paren-init28.C > create mode 100644 gcc/testsuite/g++.dg/cpp2a/paren-init29.C > create mode 100644 gcc/testsuite/g++.dg/cpp2a/paren-init30.C > create mode 100644 gcc/testsuite/g++.dg/cpp2a/paren-init31.C > create mode 100644 gcc/testsuite/g++.dg/cpp2a/paren-init32.C > > diff --git a/gcc/cp/typeck.c b/gcc/cp/typeck.c > index a557f3439a8..9166156a5d5 100644 > --- a/gcc/cp/typeck.c > +++ b/gcc/cp/typeck.c > @@ -7480,6 +7480,20 @@ build_static_cast_1 (location_t loc, tree type, tree expr, bool c_cast_p, > t. */ > result = perform_direct_initialization_if_possible (type, expr, > c_cast_p, complain); > + /* P1975 allows static_cast(42), as well as static_cast(42), > + which initialize the first element of the aggregate. We need to handle > + the array case specifically. */ > + if (result == NULL_TREE > + && cxx_dialect >= cxx20 > + && TREE_CODE (type) == ARRAY_TYPE) > + { > + /* Create { EXPR } and perform direct-initialization from it. */ > + tree e = build_constructor_single (init_list_type_node, NULL_TREE, expr); > + CONSTRUCTOR_IS_DIRECT_INIT (e) = true; > + CONSTRUCTOR_IS_PAREN_INIT (e) = true; > + result = perform_direct_initialization_if_possible (type, e, c_cast_p, > + complain); > + } > if (result) > { > if (processing_template_decl) > diff --git a/gcc/testsuite/g++.dg/cpp2a/paren-init27.C b/gcc/testsuite/g++.dg/cpp2a/paren-init27.C > new file mode 100644 > index 00000000000..0b8cbe33b69 > --- /dev/null > +++ b/gcc/testsuite/g++.dg/cpp2a/paren-init27.C > @@ -0,0 +1,24 @@ > +// PR c++/92812 > +// P1975R0 > +// { dg-do run { target c++20 } } > +// { dg-options "-Wall -Wextra" } > + > +struct Aggr { int x; int y; }; > +struct Base { int i; Base(int i_) : i{i_} { } }; > +struct BaseAggr : Base { }; > +struct X { }; > +struct AggrSDM { static X x; int i; int j; }; > + > +int > +main () > +{ > + Aggr a = static_cast(42); // { dg-warning "missing initializer" } > + if (a.x != 42 || a.y != 0) > + __builtin_abort (); > + BaseAggr b = static_cast(42); > + if (b.i != 42) > + __builtin_abort (); > + AggrSDM s = static_cast(42); // { dg-warning "missing initializer" } > + if (s.i != 42 || s.j != 0) > + __builtin_abort (); > +} > diff --git a/gcc/testsuite/g++.dg/cpp2a/paren-init28.C b/gcc/testsuite/g++.dg/cpp2a/paren-init28.C > new file mode 100644 > index 00000000000..8c57dc8e155 > --- /dev/null > +++ b/gcc/testsuite/g++.dg/cpp2a/paren-init28.C > @@ -0,0 +1,15 @@ > +// PR c++/92812 > +// P1975R0 > +// { dg-do compile { target c++20 } } > + > +// In both cases the reference declarations lifetime-extend the array > +// temporary. > +int (&&r)[3] = static_cast(42); > +int (&&r2)[1] = static_cast(42); > + > +// Make sure we've lifetime-extended. > +// { dg-final { scan-assembler "_ZGR1r_" } } > +// { dg-final { scan-assembler "_ZGR2r2_" } } > + > +// Narrowing is probably OK here. > +int (&&r3)[1] = static_cast(1.3); > diff --git a/gcc/testsuite/g++.dg/cpp2a/paren-init29.C b/gcc/testsuite/g++.dg/cpp2a/paren-init29.C > new file mode 100644 > index 00000000000..c39cd8cc0d0 > --- /dev/null > +++ b/gcc/testsuite/g++.dg/cpp2a/paren-init29.C > @@ -0,0 +1,15 @@ > +// PR c++/92812 > +// P1975R0 > +// { dg-do run { target c++20 } } > + > +int (&&r)[3] = static_cast(42); > +int (&&r2)[1] = static_cast(42); > + > +int > +main () > +{ > + if (r[0] != 42 || r[1] != 0 || r[2] != 0) > + __builtin_abort (); > + if (r2[0] != 42 || sizeof (r2) != sizeof (int)) > + __builtin_abort (); > +} > diff --git a/gcc/testsuite/g++.dg/cpp2a/paren-init30.C b/gcc/testsuite/g++.dg/cpp2a/paren-init30.C > new file mode 100644 > index 00000000000..6b2e86f00f4 > --- /dev/null > +++ b/gcc/testsuite/g++.dg/cpp2a/paren-init30.C > @@ -0,0 +1,23 @@ > +// PR c++/92812 > +// P1975R0 > +// { dg-do compile { target c++20 } } > + > +struct S1 { > + int i; > + int j; > +}; > + > +struct S2 { > + S1 s[4]; > +}; > + > +struct S3 { > + S2 s2; > +}; > + > +void > +f () > +{ > + // Brace elision not allowed. > + auto s3 = static_cast(1); // { dg-error "could not convert" } > +} > diff --git a/gcc/testsuite/g++.dg/cpp2a/paren-init31.C b/gcc/testsuite/g++.dg/cpp2a/paren-init31.C > new file mode 100644 > index 00000000000..3600c4993a5 > --- /dev/null > +++ b/gcc/testsuite/g++.dg/cpp2a/paren-init31.C > @@ -0,0 +1,10 @@ > +// PR c++/92812 > +// P1975R0 > +// { dg-do compile { target c++20 } } > +// Test we don't lifetime-extend the int temporary. > + > +struct A { const int &r; }; > +A a(42); > +auto a2 = static_cast(42); > + > +// { dg-final { scan-assembler-not "_ZGR" } } > diff --git a/gcc/testsuite/g++.dg/cpp2a/paren-init32.C b/gcc/testsuite/g++.dg/cpp2a/paren-init32.C > new file mode 100644 > index 00000000000..bf7833ebd5b > --- /dev/null > +++ b/gcc/testsuite/g++.dg/cpp2a/paren-init32.C > @@ -0,0 +1,21 @@ > +// PR c++/92812 > +// P1975R0 > +// { dg-do compile { target c++20 } } > + > +struct A { int i; }; > +struct A2 { int i; A2(int); }; > +struct A3 { int i; explicit A3(int); }; > + > +struct X { A a; }; > +auto x = static_cast(42); // { dg-error "could not convert" } > + > +struct X2 { A2 a; }; > +auto x2 = static_cast(42); > + > +struct X3 { A3 a; }; > +// Aggregate-initialization copy-initializes, so the explicit ctor > +// isn't considered. > +auto x3 = static_cast(42); // { dg-error "could not convert" } > + > +struct NonAggr { int i; virtual void foo (); }; > +auto x4 = static_cast(42); // { dg-error "no matching" } > > base-commit: ebd203778cc56df61cc3434fa01e77b4bae26362 >