> 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...

Correct me if I'm wrong, but there is no part of the C spec that says this:

When initializing a union member that is smaller than the largest member, the remaining bytes will always automatically be initialized to zero.

If I'm right then the following caveat must be added to your statement:

> A pointer to the union points to each of its members, and can be dereferenced to access it.

... if and only if the member which was originally initialized is at least as large as the other member being accessed.

In other words, if you write your program in a way that ensures it will only compile when all union members are exactly the same size, and you have mandatory tooling to make sure that any changes to said union follow the same rule by force of compilation errors, then and only then can you claim what you claimed without the threat of undefined behavior.