Bugs with opaque result types in structural position

[slavapestov]

Each of these three declarations should type check, but doesn't:

protocol P {
}

extension P {
  func f1() -> () -> (some P) {
    return { self } // error: cannot convert value of type 'Self' to closure result type 'some P'
  }

  func f2() -> () -> (some P) {
    return { () -> Self in self } // error: function declares an opaque return type, but has no return statements in its body from which to infer an underlying type
  }

  func f3() -> (some P).Type {
    return Self.self // error: function declares an opaque return type, but has no return statements in its body from which to infer an underlying type
  }
}

[slavapestov]

CC @jckarter

[vanvoorden]

extension P {
  func f1() -> () -> (some P) {
    return { self } // error: cannot convert value of type 'Self' to closure result type 'some P'
  }
}

@slavapestov It looks like f2 and f3 are both fixed for me in Swift 5.9.2… but I'm still broken on f1. Was there any roadmap to fix that? Thanks!

[vanvoorden]

//  https://github.com/apple/swift/blob/main/test/SILGen/opaque_result_type_structural.swift

public protocol P1 {}

extension P1 {
  public func f1() -> () -> (some P1) {
    let fn = { self }
    return fn
  }
  
  static public func f2() -> (Self) -> (some P1) {
    let fn: (Self) -> (Self) = { p in p }
    return fn
  }
}

public protocol P2 {}

extension P2 {
  public func f1() -> some P2 {
    return self
  }
  
  public func f2() -> some P2 {
    return self.f1()
  }
  
  public func f3() -> () -> (some P2) {
    let fn = { self.f2() }
    return fn
  }
  
  static public func f4() -> (Self) -> (some P2) {
    let fn: (Self) -> (Self) = { p in p.f2() }
    //  Cannot convert value of type 'some P2' to closure result type 'Self'
    return fn
  }
}

I was able to repurpose the workaround from opaque_result_type_structural here to unblock me on that… I'm still seeing a similar error for f4 when I don't know the concrete type… but that's kind of edge-casey and not blocking me at the moment.