enums exceeding the size of the largest number type

Your compiler is most likely using a 128 bit signed integral type as the backing type, in concurrence with the C++ standard.

See for yourself with

std::cout << sizeof(std::underlying_type<A>::type);

Link: https://ideone.com/z4K0rz, outputs 16.

The output you observe is consistent with a narrowing conversion of this to a 64 bit unsigned type.