[Bug c++/105045] New: [modules] Can't deduce defaulted template parameter

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

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=105045

            Bug ID: 105045
           Summary: [modules] Can't deduce defaulted template parameter
           Product: gcc
           Version: 12.0
            Status: UNCONFIRMED
          Keywords: rejects-valid
          Severity: normal
          Priority: P3
         Component: c++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: johelegp at gmail dot com
  Target Milestone: ---

See https://godbolt.org/z/3WMd4T48h.

`mod.cpp`:
```C++
export module mod;
export template<int T=0, class U> void f(U) { }
```

`test.cpp`:
```C++
import mod;
int main() { f(0); }
```

Output:
```
test.cpp: In function 'int main()':
test.cpp:2:15: error: no matching function for call to 'f(int)'
    2 | int main() { f(0); }
      |              ~^~~
In module mod, imported at /app/test.cpp:1:
mod.cpp:2:40: note: candidate: 'template<int T, class U> void f@mod(U)'
    2 | export template<int T=0, class U> void f(U) { }
      |                                        ^
mod.cpp:2:40: note:   template argument deduction/substitution failed:
test.cpp:2:15: note:   couldn't deduce template parameter 'T'
    2 | int main() { f(0); }
      |              ~^~~
```