[SR-14731] Self<T> is allowed in generic type, but it doesn't work as expected.

[swift-ci]

Previous ID	SR-14731
Radar	rdar://problem/79051797
Original Reporter	ensan (JIRA User)
Type	Bug
Status	Resolved
Resolution	Done

Environment

Apple Swift version 5.4 (swiftlang-1205.0.26.9 clang-1205.0.19.55)
Target: x86_64-apple-darwin20.5.0

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	None
Priority	Medium

md5: a6580430cf4875efea2b6580bfa43a4f

Issue Description:

I couldn't find previous report about this behavior. Perhaps this is a known-issue, but just in case.

struct Wrapper<A> {
    // actually WrapperInt is Wrapper<A>
    typealias WrapperInt = Self<Int>
}

I read https://bugs.swift.org/browse/SR-7666, but about this, compiler should cause at least warning, because it must not work as expected. If `Self` stands for `Wrapper`, it should work as expected. If `Self` stands for `Wrapper<A>` then it should be error, because `Wrapper<A><Int>` doesn't make sense.

[typesanitizer]

You're right, this should be an error instead of being silently ignored.

@swift-ci create

[CodaFi]

Huh, seems like we're not substituting this thing correctly

struct Wrapper<A> {
    // actually WrapperInt is Wrapper<A>
    typealias WrapperInt = Self<Int>

    func eat(_ x: WrapperInt) {
      x.foo() // error!
    }
}

extension Wrapper where A == Int {
  func foo() {}
}

[slavapestov]

Yeah, I agree that this should be an error. The original proposal did not say anything about substituting in alternate generic arguments with `Self`.

[slavapestov]

#37891