> First of all, there is no way the compiler is going to optimize a call to memset(&foo, 0, sizeof(foo)) when &foo is being interpreted as a void pointer. That doesn't even make sense.

that is really dangerous to assume. memset is a compiler built-in in every relevant C++ compiler and the compiler definitely knows the type of the object that is behind your void* and knows if you're being nasty.

e.g. look at this code : https://gcc.godbolt.org/z/xh9BXs

it's UB, and the compiler knows it and inserts an "invalid opcode" instruction even if you try to hide a memset behind a void*-taking function