[SR-2427] Cannot override generic methods in generic subclass

[DeFrenZ]

Previous ID	SR-2427
Radar	rdar://problem/18560464
Original Reporter	@DeFrenZ
Type	Bug
Status	Resolved
Resolution	Done

Additional Detail from JIRA

Votes	1
Component/s	Compiler
Labels	Bug
Assignee	@slavapestov
Priority	Medium

md5: 947d76ec0225e4dfba0192393a7153ce

is duplicated by:

• SR-2780 Cannot override generic method in generic subclass
• SR-2998 Generic methods with type constraints are inconsistently overridable

relates to:

• SR-2721 Cannot override generic methods in non-generic subclass

Issue Description:

In the following snippet I can't make `Bar.foo` an `override`, or it won't compile. And if I don't when I call it it will of course complain about ambiguousity... making the subclass or the method non-generic would make things o-kay, but the case with both seems to be not working correctly.

public class Foo {
    public func foo <T> (_ foo: T) {
        print("Foo.foo")
    }
}

public class Bar<B>: Foo {
    // can't make it an `override`
    public func foo <T> (_ foo: T) {
        print("Bar.foo")
    }
}

[belkadan]

Huh. @jckarter, @slavapestov, any ideas?

[slavapestov]

The problem here is that the generic parameter 'T' in Bar.foo has a different depth than the 'T' in Foo.foo, so the override matching code considers this a mismatch.

Of course this is wrong. If you look at how protocol requirements are matched to witnesses, the code path is totally separate and we use a mini constraint solver, 'opening up' the requirement and witness types and equating the type variables to compute substitutions. We need to re-use the same logic for overrides, or just special-case it to take the depth into account.

[slavapestov]

Merged #5424