Overloading operators of built-in types

Why is it possible to overload operators for __m128 (although Microsoft didn't defined any operators for __m128) with the MSVC++ compiler? It seems strange compared to other built-in types such as float, but these are of course part of the C++ standard whereas __m128 isn't. inline __m128 & __vectorcall operator+=(__m128 &v1, const __m128 &v2) { v1 = _mm_add_ps(v1, v2); return v1; }