Diagnose an invalid use of non-polymorphic constructors

[slavapestov]

Fixes rdar://problem/21991470, https://bugs.swift.org/browse/SR-5022.

[slavapestov]

@swift-ci Please smoke test

[jrose-apple]

Seems like we'd need to downgrade this to a warning in Swift 4 mode.

[swift-ci]

Build failed
Swift Test Linux Platform
Git Sha - 54de9a0b51488eeb44126e72d4cfe149d1804544

[slavapestov]

@jrose-apple Here is a reduced version of the example from SILGen/protocol_extensions.swift:

import Foundation
  
class ObjCInitClass {
  @objc init(s: String) { self.x = s }

  var x: String
}

protocol ProtoDelegatesToObjC { }

extension ProtoDelegatesToObjC where Self : ObjCInitClass {
  init(string: String) {
    self.init(s: string)
  }
}

class X : ObjCInitClass, ProtoDelegatesToObjC {}

print(X(string: "hello world"))

The code compiles successfully with asserts off but the print() prints out a metatype and not an instance as expected. So it's just encountering undefined behavior. If I remove the @objc, I get a bus error instead. I don't think keeping this as a warning is a good idea, since there's no vtable entry for the initializer in this case and it's unlikely anyone is relying on this with good results.

[slavapestov]

@swift-ci Please smoke test

[slavapestov]

@swift-ci Please test source compatibility

[swift-ci]

Build failed
Swift Test OS X Platform
Git Sha - 54de9a0b51488eeb44126e72d4cfe149d1804544

[jrose-apple]

Hm, okay, fair enough.