https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671
--- Comment #36 from Jason Merrill <jason at gcc dot gnu.org> --- (In reply to Richard Biener from comment #34) > 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. Nothing in the standard makes memcpy magic; 3.9/3 says, For any trivially copyable type T, if two pointers to T point to distinct T objects obj1 and obj2, where neither obj1 nor obj2 is a base-class subobject, if the underlying bytes (4.4) making up obj1 are copied into obj2, obj2 shall subsequently hold the same value as obj1. It just talks about copying the underlying bytes, not necessarily using memcpy to do so. And a copy of a B is a memberwise copy, which implies copying the U member, which as you quote means copying the object representation, which is the underlying bytes.
