https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100003
--- Comment #1 from Marek Polacek <mpolacek at gcc dot gnu.org> ---
Without the directive:
q.C:6:22: error: declaration of ‘S<T>::S() noexcept (T::value)’ has a different
exception specifier
6 | template<typename T> S<T>::S() noexcept(T::value) {}
| ^~~~
q.C:4:3: note: from previous declaration ‘S<T>::S() noexcept
(((S<T>*)this)->S<T>::foo())’
4 | S() noexcept(foo());
| ^
