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.

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