Nested generic extensions require `where` clauses instead of <…>

[JessyCatterwaul]

Given

enum Outer<O> {
  enum Inner<I> { }
}

This form compiles:

extension Outer.Inner where I == Outer { }

But this produces the error, "Reference to generic type 'Outer' requires arguments in <...>".

extension Outer.Inner<Outer> { }

[AnthonyLatsis]

I am almost certain this is intentional as per SE-0361. The extended type itself is not and was never resolved in the context of the extended type declaration, unlike the where clause:

struct S<T> {
  typealias A = Int
}
extension S where T == A {}
extension S<A> {} // error: cannot find type 'A' in scope

Doing so is a source-breaking and, in my opinion, arguable change that would need to go through the Swift evolution process. For example, would this mean the generic parameters could be referenced in the extended type as well?

extension Array<Element> {} // Okay...
extension Array<Element?> {} // What?

cc @hborla @slavapestov for more viewpoints.

[slavapestov]

You either have to specify all the generic arguments, or none of them. So this works:

extension Outer<Int>.Inner<String>

I'm not sure if we consider this a bug or working as intended though.

[JessyCatterwaul]

You either have to specify all the generic arguments, or none of them.

This is not true. E.g. these both compile:

extension Outer.Inner<Never> { }
extension Outer<Never>.Inner { }

Testing to make sure, I realized that extension Outer.Inner<Outer> not compiling is a regression. It works with Swift 5.7.

I'm working on creating another example which more closely mimics my use case; will update here when I can pinpoint exactly what breaks it. It breaks with 5.7 and 5.8. Edit: Done; see my next post.

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Doing so is a source-breaking

While your examples are different than what I'm talking about, I don't think the source can be "broken" if it didn't compile, right? A regression like this is hypothetically source-breaking, but it only matters if code was ever written that used it—it may have only been possible in the the 5.7 cycle. I may never have tried before that.