[SR-10713] The dynamic nature of a protocol's Self type isn't checked when equated with another associated type

[hamishknight]

Previous ID	SR-10713
Radar	None
Original Reporter	@hamishknight
Type	Bug
Status	Closed
Resolution	Done

Environment

Swift version 5.1-dev (LLVM 082dec2e22, Swift 397965d3fc)
Target: x86_64-apple-darwin18.5.0

Additional Detail from JIRA

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

md5: 68e52fe5d63fd695a13a186f306cb4bc

Issue Description:

The following unsound code is allowed to compile, which lets us turn a subclass instance into a superclass instance:

protocol P {
  associatedtype X where X == Self
  func foo() -> X
}

class C : P {
  typealias X = C
  func foo() -> X {
    return C()
  }
}
class D : C {}

func foo<T : P>(_ x: inout T) {
  x = x.foo()
}

var d = D()
foo(&d)
print(type(of: d)) // C

When checking the constraint X == Self, we should treat Self as being the dynamic self and therefore reject the conformance.

[belkadan]

cc @slavapestov, @DougGregor

[slavapestov]

Ouch!

[slavapestov]

I think the only safe thing to do here is to not allow a class to conform to P at all.

[belkadan]

Or only final classes, or something.

[slavapestov]

#41545