[Bug c++/94306] New: Improve diagnostic when "requires" used instead of "requires requires" and add fix-it

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=94306

            Bug ID: 94306
           Summary: Improve diagnostic when "requires" used instead of
                    "requires requires" and add fix-it
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Keywords: diagnostic
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: redi at gcc dot gnu.org
  Target Milestone: ---

template<typename T> struct S { };
template<typename T> requires { typename T::type; } struct S<T> { };

This gives:

c.cc:2:31: error: expected primary-expression before '{' token
    2 | template<typename T> requires { typename T::type; } struct S<T> { };
      |                               ^
c.cc:2:31: error: expected unqualified-id before '{' token
c.cc:2:62: error: 'T' was not declared in this scope
    2 | template<typename T> requires { typename T::type; } struct S<T> { };
      |                                                              ^
c.cc:2:63: error: template argument 1 is invalid
    2 | template<typename T> requires { typename T::type; } struct S<T> { };
      |                                                               ^
c.cc:2:53: error: an explicit specialization must be preceded by 'template <>'
    2 | template<typename T> requires { typename T::type; } struct S<T> { };
      |                                                     ^~~~~~~~~~~
      |                                                     template <> 


The first error is right. The second seems redundant with the first.

The rest of the errors seem bogus. T has been declared, the template argument
is valid, and the specialization is preceded by a template-head.

It would be good to suggest that it should say "requires requires {" and show a
fix-it hint.