Invalid monomorphization of an intrinsic function was used.

Erroneous code example:

#![feature(intrinsics)]

extern "rust-intrinsic" {
    fn simd_add<T>(a: T, b: T) -> T;
}

fn main() {
    unsafe { simd_add(0, 1); }
    // error: invalid monomorphization of `simd_add` intrinsic
}

The generic type has to be a SIMD type. Example:

#![feature(repr_simd)]
#![feature(intrinsics)]

#[repr(simd)]
#[derive(Copy, Clone)]
struct i32x2(i32, i32);

extern "rust-intrinsic" {
    fn simd_add<T>(a: T, b: T) -> T;
}

unsafe { simd_add(i32x2(0, 0), i32x2(1, 2)); } // ok!