[Bug c++/107113] New: In function parameter list, `...` expanding a pack is accepted after `()` of a parameter declarator, but not before

[iamsupermouse at mail dot ru via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107113

            Bug ID: 107113
           Summary: In function parameter list, `...` expanding a pack is
                    accepted after `()` of a parameter declarator, but not
                    before
           Product: gcc
           Version: 12.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: iamsupermouse at mail dot ru
  Target Milestone: ---

GCC rejects this code:

    template <typename ...P>
    struct A
    {
        void Foo(P...());
    };

    A<int, int> a;

But accepts this:

    template <typename ...P>
    struct A
    {
        void Foo(P()...);
    };

    A<int, int> a;

Only the first snippet should compile, since `...` must be located immediately
before a parameter name (or a place where it would otherwise be located), as
commonly seen in cases like `X &&... x`.

Clang only accepts the first snippet.