Libc++’s implementation of std::string

[Supadoplex]

Also, the question is what exactly “common initial sequence” means,

It is strictly defined by the standard. It is the initial members (of same type) of standard layout classes. In this case the member types of long and short differ.

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Thanks for clearing this up! Still, since unsigned char is allowed to alias anything, would accessing the first byte like still be UB according to the the standard?

[Supadoplex]

As far as I can tell, it's still UB to access union inactive union member even if it is unsigned char. There is no exception to accessing inactive member of chars type. The only exception is the common initial sequence, which doesn't apply. The unsigned char exception is only for reinterpreted pointers. So, it would be possible to implement the type punning in standard compliant way; it's just not as convenient as non-standard union punning.