[Bug c++/107939] New: Rejects use of `extern const` variable in a template

["johelegp at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=107939

            Bug ID: 107939
           Summary: Rejects use of `extern const` variable in a template
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: johelegp at gmail dot com
                CC: johelegp at gmail dot com
  Target Milestone: ---

See https://compiler-explorer.com/z/q3a7e7s1e.
Changing `#if 1` to `#if 0` works, so this can't be due to
> The program is ill-formed, no diagnostic required, if:
> (6.4) a hypothetical instantiation of a template immediately following its definition would be ill-formed due to a construct that does not depend on a template parameter, or
> -- https://eel.is/c++draft/temp.res.general#6.4

And since all compilers accept it as a non-template, nothing in [expr.const] is
being violated.

```C++
struct Q {
  struct P {
    const Q* p;
  };
  int n;
  constexpr P operator()(auto) const { return {this}; }
};

extern const Q q;

#if 1
template<int> constexpr auto p = q(0);
static_assert(p<0>.p == &q);
#else
constexpr auto p = q(0);
static_assert(p.p == &q);
#endif

constexpr Q q = {};
```

```
<source>:12:37: error: the value of 'q' is not usable in a constant expression
   12 | template<int> constexpr auto p = q(0);
      |                                     ^
<source>:9:16: note: 'q' was not declared 'constexpr'
    9 | extern const Q q;
      |                ^
<source>:13:22: error: non-constant condition for static assertion
   13 | static_assert(p<0>.p == &q);
      |               ~~~~~~~^~~~~
Compiler returned: 1
```