[ASTPrinter] Print the desugared constraint type following the any keyword.

[hborla]

Currently, a protocol typealias with an underlying constraint type (i.e. a protocol or protocol composition) can be used as an existential type by accessing the alias through a type parameter, e.g.

protocol P1 {
  typealias A = P2
}

protocol P2 {}

func generic<T: P1>(value: (T) -> T.A) {} // okay

However, verifying that the underlying type of T.A is a constraint type is a circular computation in the general case, which is needed to use the any keyword. Resolving T.A where T is a type parameter needs a full generic signature to access the requirements on T, but a full generic signature needs the existential type verification.

This is an issue when T.A is written in source, but any is automatically printed in Swift interfaces. For now, simply print the underlying constraint type.

Resolves: rdar://93780361

[hborla]

@swift-ci please smoke test

[hborla]

@swift-ci please smoke test

[tshortli]

My only hesitation here is that we might want to think about whether this interferes with the ability of -preserve-types-as-written to help library owners work around naming ambiguities. There are already places where library owners can't control the type names that are written into the interface, though, so maybe it's not much of a regression to lack control for this kind of signature.

[slavapestov]

I think -preserve-types-as-written will skip this code path entirely, right? It should print the TypeRepr if available (which will print T.A without any) and not look at the Type at all.

[hborla]

In any case, this isn't a long term fix; either we need to figure out how to make any with protocol typealiases work (seems unlikely), or eventually ban them in source.

[AnthonyLatsis]

By a cycle, do you mean something like this?

protocol P1 {
  associatedtype Assoc

  typealias A = P2
}
protocol P2 {}

func foo<T: P1>(t: T) where T.Assoc == any T.A {}

Sorry to bother, just trying to puzzle out some criteria for these cycles. AFAICT, they can really happen only if any T.A appears in a generic requirement.