Extensions to protocol typealias types compile but can't be accessed

[daniel-hall]

Describe the bug

If I declare a typealias for a concrete type in a protocol, e.g.

struct A { }

protocol P {
  typealias Element = A
}

and then I write an extension based on that typealias on a conforming type, e.g.:

struct Conforming: P { }

extension Conforming.Element { // This should be identical to `extension A`
  var success: Bool { true }
}

The compiler allows it. But trying to access the extension property will fail:

print(Conforming.Element().success) //Compiler error:  "Value of type 'Conforming.Element' (aka 'A') has no member 'success'"

If I declare the extension directly on the type (not through the typealias), e.g.

extension A {
  var success: Bool { true }
}

Then the same print statement (accessing type A through the protocol typealias) will succeed

Declaring the typealias directly on the type allows the extension through the typealias to work:

struct Conforming {
  typealias Element = A
}

extension Conforming.Element { 
  var success: Bool { true }
}

print(Conforming.Element().success) //  "true"

It only seems to fail on the requirement of writing an extension that refers to a typealias declared by a protocol.

Environment (please fill out the following information)

• OS: macOS 12.6
• Xcode Version/Tag/Branch: Version 14.0 (14A309)

Additional context

Ultimately what I want to accomplish is something like this (just a convenience really)

protocol SettingsProvider { 
  typealias SettingsConfiguration = Configuration<Self>
}

struct Configuration<T> { }

struct ConcreteType: SettingsProvider { }

// I want to be able to do this
extension ConcreteType.Settings {
  var someProperty: String
}

// Instead of this equivalent that seems much less intuitive to me
extension Configuration<ConcreteType> {
  var someProperty: String
}

Swift Forums thread here

[slavapestov]

This is now diagnosed on main:

q.swift:9:1: error: extension of type 'Conforming.Element' (aka 'A') must be declared as an extension of 'A'
extension Conforming.Element { // This should be identical to `extension A`
^         ~~~~~~~~~~~~~~~~~~
q.swift:9:1: note: did you mean to extend 'A' instead?
extension Conforming.Element { // This should be identical to `extension A`
^         ~~~~~~~~~~~~~~~~~~
          A

Fix was #60364