[{"id":3678900,"web_url":"http://patchwork.ozlabs.org/comment/3678900/","msgid":"<18c81c0b-da77-4423-960f-3c5e88bf336b@redhat.com>","list_archive_url":null,"date":"2026-04-17T21:57:42","subject":"Re: [PATCH] c++/reflection: support splices with CTAD [PR124706]","submitter":{"id":4337,"url":"http://patchwork.ozlabs.org/api/people/4337/","name":"Jason Merrill","email":"jason@redhat.com"},"content":"On 4/17/26 4:28 PM, Marek Polacek wrote:\n> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?\n\nOK.\n\n> -- >8 --\n> This PR points out that we don't handle a splice-type-spec that\n> designates a deducible template, which then serves as a placeholder\n> for CTAD. This is allowed by [dcl.type.simple]/3. This patch\n> fixes that problem by calling make_template_placeholder if we\n> get a deducible template.\n> \n> \tPR c++/124706\n> \n> gcc/cp/ChangeLog:\n> \n> \t* parser.cc (cp_parser_splice_type_specifier): For ctad_template_p\n> \tcall make_template_placeholder.\n> \n> gcc/testsuite/ChangeLog:\n> \n> \t* g++.dg/reflect/error10.C: Adjust dg-error.\n> \t* g++.dg/reflect/ctad1.C: New test.\n> \t* g++.dg/reflect/ctad2.C: New test.\n> ---\n> gcc/cp/parser.cc | 6 ++++++\n> gcc/testsuite/g++.dg/reflect/ctad1.C | 13 +++++++++++++\n> gcc/testsuite/g++.dg/reflect/ctad2.C | 24 ++++++++++++++++++++++++\n> gcc/testsuite/g++.dg/reflect/error10.C | 5 ++---\n> 4 files changed, 45 insertions(+), 3 deletions(-)\n> create mode 100644 gcc/testsuite/g++.dg/reflect/ctad1.C\n> create mode 100644 gcc/testsuite/g++.dg/reflect/ctad2.C\n> \n> diff --git a/gcc/cp/parser.cc b/gcc/cp/parser.cc\n> index cc59065848a..9f2641f2ab2 100644\n> --- a/gcc/cp/parser.cc\n> +++ b/gcc/cp/parser.cc\n> @@ -6258,6 +6258,12 @@ cp_parser_splice_type_specifier (cp_parser *parser)\n> if (dependent_splice_p (type))\n> return make_splice_scope (type, /*type_p=*/true);\n> \n> + /* [dcl.type.simple]/3: A type-specifier is a placeholder for a deduced\n> + class type if [...] it is of the form typename[opt] splice-specifier and\n> + the splice-specifier designates a class template or alias template. */\n> + if (ctad_template_p (type))\n> + type = make_template_placeholder (type);\n> +\n> if (!valid_splice_type_p (type))\n> {\n> if (!cp_parser_simulate_error (parser))\n> diff --git a/gcc/testsuite/g++.dg/reflect/ctad1.C b/gcc/testsuite/g++.dg/reflect/ctad1.C\n> new file mode 100644\n> index 00000000000..5358b584bc8\n> --- /dev/null\n> +++ b/gcc/testsuite/g++.dg/reflect/ctad1.C\n> @@ -0,0 +1,13 @@\n> +// PR c++/124706\n> +// { dg-do compile { target c++26 } }\n> +// { dg-additional-options \"-freflection\" }\n> +\n> +template struct X;\n> +template <> struct X {\n> + constexpr X(int) {}\n> +};\n> +X(int) -> X;\n> +\n> +static constexpr auto template_info = ^^X;\n> +\n> +[:template_info:] y{ 13 };\n> diff --git a/gcc/testsuite/g++.dg/reflect/ctad2.C b/gcc/testsuite/g++.dg/reflect/ctad2.C\n> new file mode 100644\n> index 00000000000..21f07a419b9\n> --- /dev/null\n> +++ b/gcc/testsuite/g++.dg/reflect/ctad2.C\n> @@ -0,0 +1,24 @@\n> +// PR c++/124706\n> +// { dg-do compile { target c++26 } }\n> +// { dg-additional-options \"-freflection\" }\n> +\n> +template \n> +struct A\n> +{\n> + A(T);\n> +};\n> +[:^^A:] a(42);\n> +typename [:^^A:] a2(42);\n> +\n> +template struct B { T x; };\n> +[:^^B:] b{0};\n> +typename [:^^B:] b2{0};\n> +\n> +template\n> +struct X { X(U) {} };\n> +\n> +template\n> +using Y = X;\n> +\n> +[:^^Y:] y{1};\n> +typename [:^^Y:] y2{1};\n> diff --git a/gcc/testsuite/g++.dg/reflect/error10.C b/gcc/testsuite/g++.dg/reflect/error10.C\n> index 891a643c1a0..1ac171c73cc 100644\n> --- a/gcc/testsuite/g++.dg/reflect/error10.C\n> +++ b/gcc/testsuite/g++.dg/reflect/error10.C\n> @@ -39,8 +39,7 @@ template\n> struct S {};\n> \n> void\n> -fn5 (int, typename [: ^^S :] i) // { dg-error \"expected a reflection of a type|declared\" }\n> -// { dg-message \"but .S. is a class template\" \"\" { target *-*-* } 39 }\n> +fn5 (int, typename [: ^^S :] i) // { dg-error \"template placeholder not permitted in parameter\" }\n> {\n> }\n> \n> @@ -49,6 +48,6 @@ void bar ();\n> \n> void\n> fn6 (int, typename [: ^^bar :] i) // { dg-error \"expected a reflection of a type|declared\" }\n> -// { dg-message \"but .bar. is a function template\" \"\" { target *-*-* } 48 }\n> +// { dg-message \"but .bar. is a function template\" \"\" { target *-*-* } 47 }\n> {\n> }\n> \n> base-commit: 957c1153f5eedbbab8be92c5805d2cf53a84b204","headers":{"Return-Path":"","X-Original-To":["incoming@patchwork.ozlabs.org","gcc-patches@gcc.gnu.org"],"Delivered-To":["patchwork-incoming@legolas.ozlabs.org","gcc-patches@gcc.gnu.org"],"Authentication-Results":["legolas.ozlabs.org;\n\tdkim=pass (1024-bit key;\n unprotected) header.d=redhat.com header.i=@redhat.com header.a=rsa-sha256\n header.s=mimecast20190719 header.b=id+kN6ym;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=38.145.34.32; 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