is duplicated by:

• SR-2166 Should be able to @NoEscape a optional closure
• SR-2444 @escaping failing on optional blocks
• SR-2474 closure in protocol extension implicitly captures mutating 'self'

Issue Description:

The following code snippet doesn't work anymore in the current Swift 3 snapshot:

func takes(anOptionalClosure: (() -> Void)?) {
    if let aClosure = anOptionalClosure {
        aClosure()
    }
}

func takes(aClosure: () -> Void) {
    takes(anOptionalClosure: aClosure)
}

> main.swift:18:30: Cannot convert value of type '() -> Void' to expected argument type '(() -> Void)?'

My guess is that the type checker thinks that `(() -> Void)?` is interpreted as a tuple type. But the parens are only necessary to make the closure optional. Interestingly this only happens with function types. `(String)?` e.g. type checks correctly.

The following snippet works as expected:

func takes(anOptionalClosure: (() -> Void)?) {
    if let aClosure = anOptionalClosure {
        aClosure()
    }
}

// Hint: Added two surrounding parens to the argument type
func takes(aClosure: (() -> Void)) {
    takes(anOptionalClosure: aClosure)
}