Extending inferred associated type

Using Swift
1

I recently ran into a compiler error that surprised me. Here's some code that demonstrates the issue (using Xcode 14 beta 5):

struct Test: Identifiable {
    let id: Int
}

extension Test.ID {
    static var current: Self { 0 }
}

print(Test.ID.current) // Type 'Test.ID' (aka 'Int') has no member 'current'

The compiler has inferred that type Test.ID is Int, but can't find the static value. Making the associated type explicit eliminates the error:

struct Test {
    let id: Int
}

extension Test: Identifiable {
    typealias ID = Int
}

extension Test.ID {
    static var current: Self { 0 }
}

print(Test.ID.current)

It also works if you extend Int instead of Test.ID to declare the static property. Obviously this is easy to get around, but am I wrong to assume the original example should be valid?

2

This actually gives me an error on main:

error: extension of type 'Test.ID' (aka 'Int') must be declared as an extension of 'Int'
extension Test.ID {
^         ~~~~~~~
note: did you mean to extend 'Int' instead?
extension Test.ID {
^         ~~~~~~~
          Int
error: type 'Test.ID' (aka 'Int') has no member 'current'
print(Test.ID.current)
      ~~~~~~~ ^~~~~~~

If you change the extension (as per the note) to Int then it works fine.

3

Yes, there's a couple approaches that avoid the issue, but I thought it was odd that the extension works differently when I explicitly provide a typealias vs when the compiler infers the associated type. They're functionally equivalent.