According to the standard, instantiating of non-FP complex types is an
unspecified behavior.
http://eel.is/c++draft/complex.numbers.general#2.sentence-1
Non-FP complex types happen to work with libc++, libstdc++ 8 and older, but
break with libstdc++ 9 and newer due to the library not providing implicit
conversion std::complex<FP> ones.