[SR-6480] The self value in a class method should always have DynamicSelfType

[hamishknight]

Previous ID	SR-6480
Radar	rdar://problem/35730964
Original Reporter	@hamishknight
Type	Bug

Environment

Swift version 4.1-dev (LLVM fe49d8f2ca, Clang 839070845c, Swift 76ea3a2)
Target: x86_64-apple-darwin17.2.0

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug
Assignee	@slavapestov
Priority	Medium

md5: 2df450011114c0bdf1e6f6a7bba3ac45

is duplicated by:

• SR-14899 self is not convertible to Self type

Issue Description:

The following code outputs "C":

class C {
  static var foo: String { return "\(self)" }
  let bar = foo
}

class D : C {}

print(D().bar) // C

Despite the fact that it's a D instance being constructed.

This also prevents overrides of class members from working:

class C {
  class var foo: String { return "C" }
  let bar = foo
}

class D : C {
  override class var foo: String { return "D" }
}

print(D().bar) // C

[belkadan]

Vaguely related to the lazy property problem. cc @jckarter, @slavapestov

[slavapestov]

Yes, arguably 'self' should have DynamicSelfType in all class methods, not just those returning self. Right now this cannot be enabled without introducing performance regressions, because of optimizer limitations. I'm going to re-title this bug because finally somebody cares about this issue (yay!). It's a lot more general than just properties.

[slavapestov]

@swift-ci create

[slavapestov]

Also arguably, this could use a proposal, since it's source breaking (in a very minor way).

Note that it does not impact ABI, so there's no real urgency to deal with it in the short term.

[belkadan]

I don't think the retitling is correct. If you write a static function that reads foo it does the right thing.

class C {
  static var foo: String { return "\(self)" }
  static func bar() -> String { return foo }
}

class D : C {}

print(D.bar()) // D

[slavapestov]

@belkadan Hmm. So 'let foo = bar' in type context is type checked like 'let foo = C.bar', not 'let foo = type(of: self).bar'. The latter would be more consistent behavior if we also change methods to have a dynamic 'self' type.