When this becomes an actual RFC, I’d like to see it discuss why macros cannot be used to solve the usability issue given in the Motivation section. For example, I could imagine a macro _mm_blend_ps! that can be called like this:
_mm_blend_ps!(a, b, CONSTANT);
which expands to:
_mm_blend_ps::<{CONSTANT}>(a, b);
AFAIK, this would solve the motivating issue without any language changes, provided that the documentation would point users to the macro.