[{"id":3680728,"web_url":"http://patchwork.ozlabs.org/comment/3680728/","msgid":"<0ee0590a-3c48-6588-dfdf-10ed0fd511fb@idea>","list_archive_url":null,"date":"2026-04-22T17:43:30","subject":"Re: [PATCH] c++/reflection: reflect on dependent class template\n [PR124926]","submitter":{"id":78319,"url":"http://patchwork.ozlabs.org/api/people/78319/","name":"Patrick Palka","email":"ppalka@redhat.com"},"content":"On Wed, 22 Apr 2026, Marek Polacek wrote:\n\n> Bootstrapped/regtested on x86_64-pc-linux-gnu, ok for trunk?\n> \n> -- >8 --\n> Here we issue a bogus error for\n> \n> ^^Cls::template Inner\n> \n> where Inner turns out to be a class type, but we created a SCOPE_REF\n> because we can't know in advance what it will substitute into, and\n> \n> ^^typename Cls::template Inner\n> \n> is invalid. The typename can only be used in\n> \n> ^^typename Cls::template Inner\n> \n> We're taking a reflection so both types and non-types are valid, so\n> I think we shouldn't give the error for ^^, and take the reflection\n> of the TEMPLATE_DECL.\n> \n> \tPR c++/124926\n> \n> gcc/cp/ChangeLog:\n> \n> \t* pt.cc (tsubst_qualified_id): New bool parameter. Don't give the\n> \t\"instantiation yields a type\" error when the new parameter is true.\n> \t(tsubst_expr) : Adjust the call to\n> \ttsubst_qualified_id.\n> \n> gcc/testsuite/ChangeLog:\n> \n> \t* g++.dg/reflect/dep15.C: New test.\n> ---\n> gcc/cp/pt.cc | 13 +++--\n> gcc/testsuite/g++.dg/reflect/dep15.C | 85 ++++++++++++++++++++++++++++\n> 2 files changed, 94 insertions(+), 4 deletions(-)\n> create mode 100644 gcc/testsuite/g++.dg/reflect/dep15.C\n> \n> diff --git a/gcc/cp/pt.cc b/gcc/cp/pt.cc\n> index 5d97ca7997f..f73805b2557 100644\n> --- a/gcc/cp/pt.cc\n> +++ b/gcc/cp/pt.cc\n> @@ -18238,12 +18238,14 @@ tsubst_baselink (tree baselink, tree object_type,\n> true if the qualified-id will be a postfix-expression in-and-of\n> itself; false if more of the postfix-expression follows the\n> QUALIFIED_ID. ADDRESS_P is true if the qualified-id is the operand\n> - of \"&\". NAME_LOOKUP_P is true if we intend to perform name lookup. */\n> + of \"&\". NAME_LOOKUP_P is true if we intend to perform name lookup.\n> + REFLECTING_P is true if this SCOPE_REF is an operand of ^^. */\n> \n> static tree\n> tsubst_qualified_id (tree qualified_id, tree args,\n> \t\t tsubst_flags_t complain, tree in_decl,\n> -\t\t bool done, bool address_p, bool name_lookup_p = true)\n> +\t\t bool done, bool address_p, bool name_lookup_p = true,\n> +\t\t bool reflecting_p = false)\n> {\n> tree expr;\n> tree scope;\n> @@ -18322,7 +18324,9 @@ tsubst_qualified_id (tree qualified_id, tree args,\n> else\n> \texpr = lookup_qualified_name (scope, expr, LOOK_want::NORMAL, false);\n> if (TREE_CODE (TREE_CODE (expr) == TEMPLATE_DECL\n> -\t\t ? DECL_TEMPLATE_RESULT (expr) : expr) == TYPE_DECL)\n> +\t\t ? DECL_TEMPLATE_RESULT (expr) : expr) == TYPE_DECL\n> +\t /* For ^^T::X, we'll take both types and non-types. */\n> +\t && !reflecting_p)\n> \t{\n> \t if (complain & tf_error)\n> \t {\n> @@ -23355,7 +23359,8 @@ tsubst_expr (tree t, tree args, tsubst_flags_t complain, tree in_decl)\n> \telse if (TREE_CODE (h) == SCOPE_REF)\n> \t h = tsubst_qualified_id (h, args, complain, in_decl,\n> \t\t\t\t /*done=*/true, /*address_p=*/false,\n> -\t\t\t\t /*name_lookup_p=*/false);\n> +\t\t\t\t /*name_lookup_p=*/false,\n> +\t\t\t\t /*reflecting_p=*/true);\n\nShould we just merge name_lookup_p with reflecting_p at this point?\nMore descriptive flag names are nice but having fewer flags is also nice :)\nAnd at first glance name_lookup_p doesn't seem precisely named, there\nare name lookup calls in tsubst_qualified_id that aren't suppressed by\nthat flag (such as the ones immediately before the error in question).\n\n\n> \telse\n> \t {\n> \t /* [expr.reflect] The id-expression of a reflect-expression is\n> diff --git a/gcc/testsuite/g++.dg/reflect/dep15.C b/gcc/testsuite/g++.dg/reflect/dep15.C\n> new file mode 100644\n> index 00000000000..40273955164\n> --- /dev/null\n> +++ b/gcc/testsuite/g++.dg/reflect/dep15.C\n> @@ -0,0 +1,85 @@\n> +// PR c++/124926\n> +// { dg-do compile { target c++26 } }\n> +// { dg-additional-options \"-freflection\" }\n> +\n> +template \n> +constexpr bool is_same_v = false;\n> +\n> +template \n> +constexpr bool is_same_v = true;\n> +\n> +// Class template\n> +template \n> +struct C1 {\n> + template struct Inner {};\n> +};\n> +\n> +template \n> +constexpr auto vt1 = ^^C1::template Inner;\n> +constexpr auto r1 = vt1;\n> +typename [:r1:] a;\n> +static_assert(r1 == ^^C1::template Inner);\n> +static_assert(is_same_v::Inner>);\n> +\n> +// Class template with typename\n> +template \n> +struct C6 {\n> + template struct Inner {};\n> +};\n> +\n> +template \n> +constexpr auto vt6 = ^^typename C6::template Inner;\n> +constexpr auto r6 = vt6;\n> +typename [:r6:] d;\n> +static_assert(r6 == ^^C6::template Inner);\n> +static_assert(is_same_v::Inner>);\n> +\n> +// Variable template\n> +template \n> +struct C2 {\n> + template static constexpr int Inner = 42;\n> +};\n> +\n> +template \n> +constexpr auto vt2 = ^^C2::template Inner;\n> +constexpr auto r2 = vt2;\n> +constexpr int i = template [:r2:];\n> +static_assert(i == 42);\n> +static_assert(vt2 == ^^C2::template Inner);\n> +\n> +// Function template\n> +template \n> +struct C3 {\n> + template static constexpr int Inner (T t) { return t; };\n> +};\n> +template \n> +constexpr auto vt3 = ^^C3::template Inner;\n> +constexpr auto r3 = vt3;\n> +static_assert(template [:r3:](42) == 42);\n> +static_assert(vt3 == ^^C3::template Inner);\n> +\n> +// Alias template\n> +template \n> +struct C4 {\n> + template using Inner = C4;\n> +};\n> +template \n> +constexpr auto vt4 = ^^C4::template Inner;\n> +constexpr auto r4 = vt4;\n> +typename [:r4:] b;\n> +static_assert(is_same_v>);\n> +static_assert(vt4 == ^^C4::template Inner);\n> +\n> +// Alias template with typename\n> +template struct X { };\n> +\n> +template \n> +struct C5 {\n> + template using Inner = X;\n> +};\n> +template \n> +constexpr auto vt5 = ^^typename C5::template Inner;\n> +constexpr auto r5 = vt5;\n> +typename [:r5:] c;\n> +static_assert(is_same_v>);\n> +static_assert(vt5 == ^^C5::template Inner);\n> \n> base-commit: cb70dab0e4bfa94e009700086001a5b2dc17ea5d\n> -- \n> 2.53.0\n> \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=HfsuirYe;\n\tdkim-atps=neutral","legolas.ozlabs.org;\n spf=pass (sender SPF authorized) smtp.mailfrom=gcc.gnu.org\n (client-ip=2620:52:6:3111::32; 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