[Bug c++/103110] New: templated operator auto is ignored

[fchelnokov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103110

            Bug ID: 103110
           Summary: templated operator auto is ignored
           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 should be valid:
```
struct S {
    template<class=void>
    operator auto() const { return 2; }
};

int main() {
    S s;
    [[maybe_unused]] int d = s;
}
```
and Clang accepts it, while GCC does not. Demo:
https://gcc.godbolt.org/z/rMK4zboEd