[{"id":3677386,"web_url":"http://patchwork.ozlabs.org/comment/3677386/","msgid":"","list_archive_url":null,"date":"2026-04-14T20:08:17","subject":"Re: [PATCH] c++: identity of A<^^alias> [PR124544]","submitter":{"id":4337,"url":"http://patchwork.ozlabs.org/api/people/4337/","name":"Jason Merrill","email":"jason@redhat.com"},"content":"On 4/14/26 3:54 PM, Patrick Palka wrote:\n> Tested on x86-64-pc-linux-gnu, does this look OK for trunk?\n> \n> -- >8 --\n> \n> Since a reflection of an alias represents the alias itself, not the\n> underlying type, equality of two such reflections needs to take that\n> into account. And don't strip such aliases when forming a\n> specialization A<^^alias>.\n> \n> \tPR c++/124544\n> \n> gcc/cp/ChangeLog:\n> \n> \t* tree.cc (strip_typedefs_expr) : Don't strip\n> \ttypedefs within a type operand.\n> \t(cp_tree_equal) : For a reflection of an\n> \talias, also compare TYPE_NAMEs directly.\n> \n> gcc/testsuite/ChangeLog:\n> \n> \t* g++.dg/reflect/alias1.C: New test.\n> \t* g++.dg/reflect/alias2.C: New test.\n> ---\n> gcc/cp/tree.cc | 22 ++++++++++++++++++----\n> gcc/testsuite/g++.dg/reflect/alias1.C | 26 ++++++++++++++++++++++++++\n> gcc/testsuite/g++.dg/reflect/alias2.C | 17 +++++++++++++++++\n> 3 files changed, 61 insertions(+), 4 deletions(-)\n> create mode 100644 gcc/testsuite/g++.dg/reflect/alias1.C\n> create mode 100644 gcc/testsuite/g++.dg/reflect/alias2.C\n> \n> diff --git a/gcc/cp/tree.cc b/gcc/cp/tree.cc\n> index ff2e78cd89cf..d12ebc0f25e1 100644\n> --- a/gcc/cp/tree.cc\n> +++ b/gcc/cp/tree.cc\n> @@ -2155,6 +2155,11 @@ strip_typedefs_expr (tree t, bool *remove_attributes, unsigned int flags)\n> case STMT_EXPR:\n> return t;\n> \n> + case REFLECT_EXPR:\n> + /* ^^alias represents the alias itself, not the underlying type. */\n> + if (TYPE_P (TREE_OPERAND (t, 0)))\n> +\treturn t;\n> +\n> default:\n> break;\n> }\n> @@ -4510,10 +4515,19 @@ cp_tree_equal (tree t1, tree t2)\n> return true;\n> \n> case REFLECT_EXPR:\n> - if (!cp_tree_equal (REFLECT_EXPR_HANDLE (t1), REFLECT_EXPR_HANDLE (t2))\n> -\t || REFLECT_EXPR_KIND (t1) != REFLECT_EXPR_KIND (t2))\n> -\treturn false;\n> - return true;\n> + {\n> +\ttree h1 = REFLECT_EXPR_HANDLE (t1);\n> +\ttree h2 = REFLECT_EXPR_HANDLE (t2);\n> +\tif (!cp_tree_equal (h1, h2)\n> +\t || REFLECT_EXPR_KIND (t1) != REFLECT_EXPR_KIND (t2))\n\nWhile we're touching this, let's compare KIND before HANDLE. OK with \nthat change.\n\n> +\t return false;\n> +\t/* ^^alias represents the alias itself, not the underlying type. */\n> +\tif (TYPE_P (h1)\n> +\t && (typedef_variant_p (h1) || typedef_variant_p (h2))\n> +\t && TYPE_NAME (h1) != TYPE_NAME (h2))\n> +\t return false;\n> +\treturn true;\n> + }\n> \n> default:\n> break;\n> diff --git a/gcc/testsuite/g++.dg/reflect/alias1.C b/gcc/testsuite/g++.dg/reflect/alias1.C\n> new file mode 100644\n> index 000000000000..d77fd9f2a978\n> --- /dev/null\n> +++ b/gcc/testsuite/g++.dg/reflect/alias1.C\n> @@ -0,0 +1,26 @@\n> +// PR c++/124544\n> +// { dg-do compile { target c++26 } }\n> +// { dg-additional-options \"-freflection\" }\n> +\n> +using alias = int;\n> +template using alias_t = int;\n> +\n> +template struct A { };\n> +static_assert( ! __is_same(A<^^int>, A<^^alias>) );\n> +static_assert( ! __is_same(A<^^int>, A<^^alias_t>) );\n> +static_assert( ! __is_same(A<^^alias>, A<^^alias_t>) );\n> +static_assert( ! __is_same(A<^^alias_t>, A<^^alias_t>) );\n> +static_assert( __is_same(A<^^alias_t>, A<^^alias_t>) );\n> +\n> +template struct B { };\n> +static_assert( __is_same(A<^^B>, A<^^B>) );\n> +\n> +template int V;\n> +static_assert( &V<^^int> != &V<^^alias_t> );\n> +static_assert( &V<^^int> != &V<^^alias> );\n> +static_assert( &V<^^alias_t> != &V<^^alias_t> );\n> +\n> +template void f();\n> +static_assert( &f<^^int> != &f<^^alias_t> );\n> +static_assert( &f<^^int> != &f<^^alias> );\n> +static_assert( &f<^^alias_t> != &f<^^alias_t> );\n> diff --git a/gcc/testsuite/g++.dg/reflect/alias2.C b/gcc/testsuite/g++.dg/reflect/alias2.C\n> new file mode 100644\n> index 000000000000..c32a8724a9ba\n> --- /dev/null\n> +++ b/gcc/testsuite/g++.dg/reflect/alias2.C\n> @@ -0,0 +1,17 @@\n> +// PR c++/124544\n> +// { dg-do compile { target c++26 } }\n> +// { dg-additional-options \"-freflection\" }\n> +\n> +template struct A;\n> +\n> +template\n> +struct A {\n> + static void f();\n> +};\n> +\n> +template using alias_t = T;\n> +\n> +int main() {\n> + A<^^int, char>::f();\n> + A<^^alias_t, char>::f();\n> +}","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=QVV+K5YM;\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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