[Bug c++/100997] New: [c++20] parse error when id-expression of constructor is simple-template-id

[felix.morgner at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100997

            Bug ID: 100997
           Summary: [c++20] parse error when id-expression of constructor
                    is simple-template-id
           Product: gcc
           Version: 11.1.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: felix.morgner at gmail dot com
  Target Milestone: ---

Created attachment 50972
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50972&action=edit
Example source file

In C++20 mode, GCC 11.1 rejects the following code:

  template<typename>
  struct A;

  template<>
  struct A<int> {
    A<int>() {}
  };

With the following error:

  <source>:6:12: error: expected unqualified-id before ')' token

I currently don't have access to my copy of the standard, but working from both
https://wg21.link/std and https://wg21.link/std20, I am under the expression
that this code should be valid since:

According to [class.ctor], the sole id-expression of the ptr-declarator of the
ctors function declarator may be the injected-class-name. [class.pre] tells us
that the class-name of the class being declared is injected into scope of the
class, where it is called the injected-class-name. Moreover, [class.pre]
defines a class-name to be either an indentifier or a simnple-template-id. I
would argue that in the case of the explicit specialization above, A<int> would
be a simple-template-id and thus a valid injected-class-name.

The error does not occur in C++17 mode or in GCC prior to version 11.

Tested in all cases with "-Wall -Wextra -Werror -pedantic-errors"