However, the way we usually handle enum optimization with extra inhabitants is problematic for floats. We normally say that it is undefined behavior for a value to have an extra inhabitant representation—a class reference cannot be null, a Bool can only be 0 or 1, and so on. With floats, we need to interoperate with numerics code not written in Swift, and we want to be able to read floating-point data out of memory that may use arbitrary bit patterns. We don't want every double-returning C function or load from memory to require a check for reserved values afterward. Making it undefined behavior for floats to have "extra inhabitant" representations is thus probably not practical.
Instead of saying that extra inhabitants are undefined behavior, we could instead continue to allow Floats and Doubles to have arbitrary bit patterns, and only check for reserved values at the point we construct an enum that wants to use reserved values for layout. If we reserve negative NaNs, then for example, constructing a Float? or Double? from a nonoptional value would check whether the payload value is NaN and if so, clear the sign bit at that point. That way, we don't have any ABI problems with Floats and Doubles from foreign sources, but still get the benefits of layout optimization for Swift types. On the other hand, this would mean that supposedly-idempotent operations like '.some(x)!' lose the sign information for NaNs. Since we wouldn't want to prevent the optimizer from folding those kinds of operations away, we could define Swift's semantics to say that querying the sign of a NaN value produces an unspecified value. This matches the intent of IEEE 754, and shouldn't impact most numerics code in practice. If we were interested in pursuing enum layout optimization with float payloads, I think this would be the best approach.