[Bug c++/70248] constexpr initialization with unspecified equality expression accepted

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70248

--- Comment #3 from Jonathan Wakely <redi at gcc dot gnu.org> ---
GCC trunk (8.0.1 20180410) now rejects comment 0 with:

c0.cc:4:24: error: a reinterpret_cast is not a constant expression
 constexpr int A::*bx = (int(A::*))&B::x;
                        ^~~~~~~~~~~~~
c0.cc:5:24: error: a reinterpret_cast is not a constant expression
 constexpr int A::*cx = (int(A::*))&C::x;
                        ^~~~~~~~~~~~~