[Bug c++/102194] New: Incorrect explicit instantiation of constexpr variable accepted

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102194

            Bug ID: 102194
           Summary: Incorrect explicit instantiation of constexpr variable
                    accepted
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fchelnokov at gmail dot com
  Target Milestone: ---

This incorrect instantiation is accepted by GCC
```
template<int N> constexpr long long fib = fib<N-1> + fib<N-2>;
template<> constexpr long long fib<1> = 1;
template<> constexpr long long fib<2> = 1;
template long long fib<70>; // error here, `const` is missing
```

The correct last line must be:
```
template const long long fib<70>;
```