https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94057
--- Comment #6 from CVS Commits <cvs-commit at gcc dot gnu.org> --- The master branch has been updated by Marek Polacek <mpola...@gcc.gnu.org>: https://gcc.gnu.org/g:517f5356bb0ca717f51e937be880e38a828edbf7 commit r10-7403-g517f5356bb0ca717f51e937be880e38a828edbf7 Author: Marek Polacek <pola...@redhat.com> Date: Wed Mar 18 19:28:14 2020 -0400 c++: DR1710, template keyword in a typename-specifier [PR94057] Consider template <typename T> class A { template <typename U> class B { void fn(typename A<T>::B<U>); }; }; which is rejected with error: 'typename A<T>::B' names 'template<class T> template<class U> class A<T>::B', which is not a type whereas clang/icc/msvc accept it. "typename A<T>::B<U>" is a typename-specifier. Sadly, our comments don't mention it anywhere, because the typename-specifier wasn't in C++11; it was only added to the language in N1376. Instead, we handle it as an elaborated-type-specifier (not a problem thus far). So we get to cp_parser_nested_name_specifier_opt which has a loop that breaks if we don't see a < or ::, but that means we can -- tentatively -- parse even B<U> which is not a nested-name-specifier (it doesn't end with a ::). I think this should compile because [temp.names]/4 says: "In a qualified-id used as the name in a typename-specifier, elaborated-type-specifier, using-declaration, or class-or-decltype, an optional keyword template appearing at the top level is ignored.", added in DR 1710. Also see DR 1812. This issue on its own is not a significant problem or a regression. However, in C++20, the typename here becomes optional, and so this test is rejected in C++20, but accepted in C++17: template <typename T> class A { template <typename U> class B { void fn(A<T>::B<U>); }; }; Here we morph A<T>::B<U> into a typename-specifier, but that happens in cp_parser_simple_type_specifier and we never handle it as above. To fake the template keyword I'm afraid we need to use cp_parser_template_id with template_keyword_p=true as in the patch below. The tricky thing is to avoid breaking concepts. To handle DR 1710, I made cp_parser_nested_name_specifier_opt assume that when we're naming a type, the template keyword is present, too. That revealed a bug: DR 1710 also says that the template keyword can be followed by an alias template, but we weren't prepared to handle that. alias-decl?.C exercise this. gcc/cp: DR 1710 PR c++/94057 - template keyword in a typename-specifier. * parser.c (check_template_keyword_in_nested_name_spec): New. (cp_parser_nested_name_specifier_opt): Implement DR1710, optional 'template'. Call check_template_keyword_in_nested_name_spec. (cp_parser_simple_type_specifier): Assume that a < following a qualified-id in a typename-specifier begins a template argument list. gcc/testsuite: DR 1710 PR c++/94057 - template keyword in a typename-specifier. * g++.dg/cpp1y/alias-decl1.C: New test. * g++.dg/cpp1y/alias-decl2.C: New test. * g++.dg/cpp1y/alias-decl3.C: New test. * g++.dg/parse/missing-template1.C: Update dg-error. * g++.dg/parse/template3.C: Likewise. * g++.dg/template/error4.C: Likewise. * g++.dg/template/meminit2.C: Likewise. * g++.dg/template/dependent-name5.C: Likewise. * g++.dg/template/dependent-name7.C: New test. * g++.dg/template/dependent-name8.C: New test. * g++.dg/template/dependent-name9.C: New test. * g++.dg/template/dependent-name10.C: New test. * g++.dg/template/dependent-name11.C: New test. * g++.dg/template/dependent-name12.C: New test. * g++.dg/template/dependent-name13.C: New test. * g++.dg/template/dr1794.C: New test. * g++.dg/template/dr314.C: New test. * g++.dg/template/dr1710.C: New test. * g++.dg/template/dr1710-2.C: New test. * g++.old-deja/g++.pt/crash38.C: Update dg-error.
