Issue Description:

Consider the following method:

struct T<A> {
  func run<C>(_ c: C) where C: Collection, C.SubSequence == A {
    print(c[...] as! A)
  }
}

This should restrict the collection passed into `run` to be such that its subsequence matches the generic A.

For example, these following compile just fine:

// These should compile ✅
T<Substring>().run("")
T<Data>().run(Data())

However, there seem to be some situations where we can pass in a collection that does not match this constraint and yet it still compiles, and hence crashes at runtime:

// These should not but do⁉️
T<Substring>().run(Data())
T<Data>().run("")