https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65892
--- Comment #16 from Jonathan Wakely <redi at gcc dot gnu.org> --- Message [c++std-core-20893] on the C++ core reflector on 2011-12-14 supports the GCC view that a C++ compiler can apply strict aliasing rules to p1->m and p2->m unless the fact they come from the same object is visible to the compiler. <quote> | Either I'm mistaken, or it is a 'magical property of unions'. If the | compiler can figure out two objects didn't come form the same union, | it can assume they do not alias, Yes. I believe I filled a core issue regarding this -- coming from controversy originating from optimizations performed by compilers today (especially without full program analysis) -- and the resolution is that if intend to play type punning games with X and Y you better let the type checker and optimizer see your cards upfront. Otherwise, you are on your own. I can't remember the issue number right now. In the end, I think that was the right decision. </quote> But I can't find the core issue referred to.