I want to assure you that the compiler does not exclusively use the ISO language standard to define the behavior of internal components of the implementation.
And even that is only if the bug report is correct.
My first post is generic and not about this particular issue; it's still fine.
For example, Darwin's libc offers this contract for memcpy and clang is perfectly within its rights to generate such code (note that the IR it generates is still violating LLVM's contract on the memcpy intrinsic); glibc offers no such contract for memcpy, and so clang's code is nonconformant.