[SR-11417] Failure to infer associated type

[jeremyabannister]

Previous ID	SR-11417
Radar	rdar://problem/55114221
Original Reporter	@jeremyabannister
Type	Bug

Additional Detail from JIRA

Votes	0
Component/s	Compiler
Labels	Bug, AssociatedTypeInference
Assignee	None
Priority	Medium

md5: 001072ec536c5ffe7ab2fefe44ba1db9

Issue Description:

This code fails to compile with swift 5:

protocol A {
    associatedtype Value
}

protocol B: A {
    func f (x: Any?) -> Value?
}

struct C: B {
    func f (x: Any?) -> String? {
        return nil
    }
}

The error is:

Type 'C' does not conform to protocol 'A'

Simply adding this:

typealias Value = String

to C fixes error. I was very surprised about this, as this seems like quite a basic type inference situation. Am I missing something or is this simply a bug?

[belkadan]

cc @DougGregor

[belkadan]

@swift-ci create