[Bug c++/77812] [5/6/7 Regression] incorrectly rejects valid C++ code that uses enum in template instantiation

[nathan at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77812

--- Comment #3 from Nathan Sidwell <nathan at gcc dot gnu.org> ---
The struct variant only 'works' because 'struct X' is a valid incomplete struct
declaration.  Try:

struct f {};

template <typename>
void f ()
{
  struct f Q;
}

template void f<int> ();

 void f() void f() [with <template-parameter-1-1> = int]
../../../77812-struct.ii: In instantiation of 'void f() [with
<template-parameter-1-1> = int]':
../../../77812-struct.ii:9:23:   required from here
../../../77812-struct.ii:6:12: error: 'f() [with <template-parameter-1-1> =
int]::f Q' has incomplete type
   struct f Q;
            ^