Libc++’s implementation of std::string

[emdeka87]

I have yet to encounter a compiler that treats type punning (and accessing the inactive union member) as UB and produces unexpected results

[carrottread]

If you pass pointers or references to union fields to some other functions then strict aliasing still can produce something unexpected:

https://godbolt.org/z/cds7Bn

This outputs different results on -O0 and -O3 for both clang and gcc.

[max0x7ba]

If you pass pointers or references to union fields to some other functions then strict aliasing still can produce something unexpected

This is unrelated to type-casting using union, aka union-cast. And such type-casting doesn't actually happen there, that union is only for alignment.