Undefined behavior != implementation defined.

Also, is it that much harder to do this (C++ for example):

    struct S {
        char c = 0;
        int i = 0;
        bool operator==(const S &other) const {
            return c == other.c && i == other.i;
        }
    };
    
    S a, b;
    if(something) return !(a == b);
    return 3;

Bam, no undefined behavior and you don't have to remember to manually zero out the structure everywhere.

If you're using memcmp on an object, you should be asserting that std::has_unique_object_representations<T> is true for it (unfortunately this is only available after C++17).

Also, if you are okay with anonymous field names:

    std::tuple<char, int> a, b;
    if(something) return a != b;
    return 3;