In theory this is a source break if someone had a weird custom
conforming type, but I suspect in practice conformances to
this protocol are never defined.
The reason we want this requirement is that often you will see
code like the following:
protocol Point {
associatedtype Scalar: SIMDScalar
associatedtype Vector: SIMD where Vector.Scalar == Scalar
}
extension Point where Vector == SIMD2<Scalar> { ... }
When `Vector` is equated with `SIMD2<Scalar>`, we get an infinite
sequence of implied same-type requirements:
Vector.MaskStorage == SIMD2<Scalar.MaskScalar>
Vector.MaskStorage.MaskStorage == SIMD2<Scalar.MaskScalar.MaskScalar>
...
The protocol fails to typecheck with an error because the requirement
machine cannot build a rewrite system.
If SIMDScalar requires that MaskScalar.MaskScalar == MaskScalar, then
we instead get
Vector.MaskStorage == SIMD2<Scalar.MaskScalar>
Vector.MaskStorage.MaskStorage == SIMD2<Scalar.MaskScalar>
Vector.MaskStorage.MaskStorage == Vector.MaskStorage
...
Which ties off the recursion.
In theory, a more advanced implementation could represent this kind of
infinite recursion in 'closed form', but we don't have that yet, and I
believe adding this same-type requirement makes sense anyway.
Fixes rdar://problem/95075552.
