Add a test for using a nested type for a base class generic argument

[jrose-apple]

This used to cause an infinite loop in the compiler but now works fine.

SR-9160

[jrose-apple]

@swift-ci Please smoke test and merge

[Sajjon]

@jrose-apple Nice! Only just four hours ago I found a related regression bug. Give me a couple of minutes and I will paste relevant code. Regression as in code that worked fine with Swift 4.2 but not with Swift 5.0

[Sajjon]

Sorry, I gave it a go but did not manage to reproduce using a minimum example.

The issue involves something like this:

protocol FoobarProtocol {
    associatedtype Foo
    func createFoo() -> Foo
}

class AbstractClass<FooInAbstract>: FooBarProtocol {
    func createFoo() -> FooInAbstract { fatalError("impl me") }
}

class BaseClass<FooInBase>: AbstractClass<FooInBase> {
  // Other base functions
}

final class ConcreteClass: BaseClass<Foo> {
    struct Foo {}
    override func createFoo() -> Foo { return Foo() }
} 

Something like that worked fine with Swift 4.2 but causing issues in Swift 5. I think it was when I referenced the type Foo that I got issues with Swift 5, where it failed to disambiguate between the declared associatedtype Foo and the concrete nested struct Foo inside ConcreteClass.

(For sake of completeness for future readers it is in my app Zhip (which is open sourcee)) in the branch swift5 where I discovered this regresssion bug)

[jrose-apple]

Please file a bug, even if you just link to a specific commit of your project!

[jrose-apple]

@swift-ci Please smoke test macOS