relates to:

• SR-5016 Swift 4: Unable to satisfy associated type requirements implicitly
• SR-5104 Protocol's associatedtype not correctly inferred when the type is a closure argument

Issue Description:

I've experienced a problem during Swift 3.1 to Swift 4 codebase migration.

The problem arrises when I try to implement a generic protocol method that takes a closure with a generic parameter with a protocol as an associated type.

The following code works fine in Swift 3.1:

protocol FooType {
    associatedtype BarType

    func foo(bar: BarType)
    func foo(action: (BarType) -> Void)
}

protocol Bar {}

class Foo: FooType {
    typealias BarType = Bar

    // Compiles in both 3.1 and 4
    func foo(bar: Bar) {
    }

    // Swift 4 error: Candidate has non-matching type (Bar) -> Void
    func foo(action: (Bar) -> Void) {     
    }
}

However in Swift 4 compiler gives me an error about class Foo not conforming to protocol FooType with foo(action: ) method implementation missing.

By the way Xcode 9 "fix-it" generates the same implementation I have.

It's possible to workaround the issue by removing the line

typealias BarType = Bar

which is ok since type inference does its job, but nevertheless it should be a valid code.