https://gcc.gnu.org/bugzilla/show_bug.cgi?id=122958
--- Comment #9 from Egas Ribeiro <egas.g.ribeiro at tecnico dot ulisboa.pt> ---
this example (actually) has the same backtrace:
```
template <class>
class C;
template <class R, class S>
class C<R(S)>;
template <class R, class S, class Extra>
class C<R(S)> {};
C<void(int)> c;
```
the issue was with maybe_new_partial_specialization, it wasn't comparing the
template heads as is specified here:
https://eel.is/c++draft/temp#spec.partial.general-2
i'll submit a patch later for this (so far had no regressions)
