Hi, The following snippet fails with gcc trunk and with gcc 9 if I compile with -std=gnu++20, but not with -std=gnu++17. gcc <= 8 or any clang version happily compiles it whatever -std value I use. template <typename T> class A { template <typename... Types> class B { B(const A<T>::B<Types...>&) = default; }; }; It reads: <source>:5:23: error: 'typename A<T>::B' names 'template<class T> template<class ... Types> class A<T>::B', which is not a type 5 | B(const A<T>::B<Types...>&) = default; | ^~~~~~~~~~~ <source>:5:34: error: expected unqualified-id before '&' token 5 | B(const A<T>::B<Types...>&) = default; | ^ <source>:5:34: error: expected ')' before '&' token 5 | B(const A<T>::B<Types...>&) = default; | ~ ^ | ) <source>:5:34: error: constructors may not be ref-qualified <source>:5:34: error: expected ';' at end of member declaration 5 | B(const A<T>::B<Types...>&) = default; | ^ | ; <source>:5:35: error: expected unqualified-id before ')' token 5 | B(const A<T>::B<Types...>&) = default; | ^ Compiler returned: 1 Cheers, Romain
Started with r9-4536-g96c35892bea68ac180f19079cf08fea3b6cda0a8
GCC 9.3.0 has been released, adjusting target milestone.
Not relate to parameter packs. Also happens with normal member functions: template <typename T> class A { template <typename U> class B { B(A<T>::B<U>&); void fn(A<T>::B<U> &); }; };
The root cause isn't the C++20 feature it seems. The following version with explicit 'typename' is rejected, but compiles fine with clang/icc: template <typename T> class A { template <typename U> class B { B(typename A<T>::B<U>&); void fn(typename A<T>::B<U> &); }; };
(We accept it when the template keyword is added: "typename A<T>::template B<U>".)
The master branch has been updated by Marek Polacek <mpolacek@gcc.gnu.org>: https://gcc.gnu.org/g:517f5356bb0ca717f51e937be880e38a828edbf7 commit r10-7403-g517f5356bb0ca717f51e937be880e38a828edbf7 Author: Marek Polacek <polacek@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.
Fixed. I don't think I want to backport this one. To make it work with G++ 9, add the template keyword.
*** Bug 65943 has been marked as a duplicate of this bug. ***
*** Bug 84186 has been marked as a duplicate of this bug. ***