The post is about C, not C++. My comment stands, as the original post has two structs in a union, and they start the same way, so it’s exactly the case covered in the C11 Standard.
It's actually weirder than that. The C standard allows type punning through unions, but not because of the clause you mentioned. It allows it because of footnote 95:
> If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called ‘‘type punning’’)
This is broader than the common initial subsequence clause, and allows punning between completely different types, e.g. int, char[4], and float.
You might ask, what is the point of the "common initial subsequence" rule then? It's to allow certain accesses that don't go directly through the union, so the compiler doesn't know for sure whether there's a union involved. Only problem is that all major compilers completely ignore this rule. [1] (But they do implement the first clause I mentioned, where the accesses do go through the union.)
> If the member used to read the contents of a union object is not the same as the member last used to store a value in the object, the appropriate part of the object representation of the value is reinterpreted as an object representation in the new type as described in 6.2.6 (a process sometimes called ‘‘type punning’’)
This is broader than the common initial subsequence clause, and allows punning between completely different types, e.g. int, char[4], and float.
You might ask, what is the point of the "common initial subsequence" rule then? It's to allow certain accesses that don't go directly through the union, so the compiler doesn't know for sure whether there's a union involved. Only problem is that all major compilers completely ignore this rule. [1] (But they do implement the first clause I mentioned, where the accesses do go through the union.)
[1] https://stackoverflow.com/questions/34616086/union-punning-s...