The spec text talks about SIMD objects and SIMD values, where a SIMD object is an object with an internal [[SIMDData]] slot holding a SIMD value. The section about the SIMD constructor prototype talks about a [[SIMDWrapperData]] slot. I assume that is that same thing?

Consider this code:

x = SIMD.Int32x4(1,2,3,4);
y = Object(x);
typeof y;                // "object"
y.toString();            // "SIMD.Int32x4(1, 2, 3, 4)".
typeof x;                // "int32x4"
x.toString();            // TypeError!

The last line will throw a TypeError because the SIMD.Int32x4.prototype.toString method requires a SIMD object. I wonder if that is intentional?

Compare to the Symbol.prototype.toString method which accepts both a symbol value and an object wrapping a symbol.

Something similar is going on with the valueOf and toLocaleString methods.