[Bug c++/126654] New: error: '::' is not a class, namespace, or enumeration

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126654

            Bug ID: 126654
           Summary: error: '::' is not a class, namespace, or enumeration
           Product: gcc
           Version: 17.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: eczbek.void at gmail dot com
  Target Milestone: ---

https://godbolt.org/z/1Tcz9sMes

```
struct S {};

template<auto ns>
void f() {
        using [:ns:]::S;
}

int main() {
        f<^^::>();
}
```

```
<source>: In instantiation of 'void f() [with auto ns = ^^::]':
<source>:9:9:   required from here
    9 |         f<^^::>();
      |         ~~~~~~~^~
<source>:5:23: error: '::' is not a class, namespace, or enumeration
    5 |         using [:ns:]::S;
      |                       ^
```