[Bug c++/127627] What's the right overload resolution for ``public: X(const X&); private: explicit X(const X&) requires T::value = default;``?

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127627

--- Comment #2 from Jonathan Wakely <redi at gcc dot gnu.org> ---
It isn't shown because the more constrained one replaces it. There is only one
copy constructor, decided by evaluating the constraints. The unconstrained one
is not an "eligible special member function" in the terminology of the
standard.

EDG and MSVC accept this too, but I don't understand why. GCC seems correct to
me, the only eligible copy constructor is the private explicit one, and so that
is chosen by overload resolution, but that's ill-formed because it's not
accessible and is explicit.