[Pitch] Introduce existential `any`

Evolution Pitches
192

Unless you are talking about changing the behavior of type(of:), that does not work and cannot work. There is no type expression that can possibly represent "the type produced by type(of:) on a T", because type(of:) does not respect generic substitution.

Suppose that such an expression existed. To try to avoid any confusion about any, let me spell it TypeOf<T>.

  • TypeOf<any Printable> is any Printable.Type, the existential metatype.
  • In a context generic over U, TypeOf<U> is U.Type.
  • If I substitute U=any Printable into U.Type, I get (any Printable).Type, the non-existential metatype.

Or in terms of values, if I have something I know statically is an any Printable, type(of:) will produce the dynamic type of the value in the existential, packaged as an existential metatype. If I have something that I don't know statically is an any Printable, but has an opaque type that dynamically happens to be any Printable, type(of:) will produce the singleton metatype value (any Printable).Type.

If Swift gains a different feature for extracting the dynamic type which respects generic substitution, then we could also add a type expression for its result type. However, I think it would be exceptionally confusing to spell that something like any T.Type, because it makes a very subtle distinction between very similar-looking types, and because the transformation it performs is subtly bound up with the any type operator.