https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79671
--- Comment #38 from Richard Biener <rguenth at gcc dot gnu.org> --- Simplified testcase for discussion (is not "miscompiled"): struct S { union { int i; } u; }; int main() { S s; new (&s.u.i) float (2.0); S q = s; if (*reinterpret_cast<float *>(&q.u.i) != 2.0) abort (); } so you say q = s is valid because the object representation of the union is copied. I say after storing 2.0 to s.u.i the access 's' in q = s is invalid as you are accessing the stored value (a float) via a glvalue of inappropriate type.