https://gcc.gnu.org/bugzilla/show_bug.cgi?id=101061
--- Comment #13 from Alexander Grund <alexander.gr...@tu-dresden.de> --- > But what you can see is that the resulting pointer is used for the > initialization and not the placement address as literally written in source. So I assume it was supposed to be "Y::Y (D_6557, 1);" ? > I'm not sure how one can solve this issue with using placement new > but are unions not sufficiently restricted so that copy assignment > should work (and activate the appropriate union member)? Thus > > slot->mutable_value = pair<const K, V>(k, v); The problem is not the copy, the problem is that the value may contain any kind of data, think e.g. a pair of strings. And at the initial point (i.e. first emplace) the slot is a casted pointer into uninitialized data. I.e. the above would be an assignment into an object which does not exist. And (especially) for such non-trivial types this would break. I think it will work for trivial types though, although it is UB due to lifetime rules: You can't use an object (here: assign to) which has not started its lifetime yet. However e.g. pair has custom copy and regular constructors so I think it will run into the issue you mentioned: The ctor will access the object via the this-pointer and not via the full union-thing and hence might misoptimise later This would mean that in conclusion the use case of putting std::pairs in an union and accessing them via aliasing is unsupported by (at least) GCC. Is that correct?
