https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671
--- Comment #34 from Richard Biener <rguenth at gcc dot gnu.org> --- C++14 12.8/16 says "The implicitly-defined copy/move constructor for a union X copies the object representation (3.9) of X." 3.9/4 says "The object representation of an object of type T is the sequence of N unsigned char objects... For trivially copyable types, the value representation is a set of bits in the object representation that determines a value,..." this suggests that the copying should work but the C++ FE may not simply elide the copy construction by emitting c = *p; because that does _not_ implement memcpy semantics for the union member. Note the above may not apply at all here if B is POD and thus the assignment is an assignment of PODs (I don't know all of the standard).
