[Bug c++/103659] New: Declared function template can be deleted later

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103659

            Bug ID: 103659
           Summary: Declared function template can be deleted later
           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 code
```
template<class T>
int foo(T);

template int foo(int);

template<class T>
int foo(T) = delete;
```
is currently accepted by GCC, but it must not by at least two reasons:
* explicit instantiation of undefined function;
* deletion of already declared function.

Demo: https://gcc.godbolt.org/z/EW8fvba5f