From the playground:

func performAction(closure: (Int) -> Void) {
    closure(7)
}

func performAction(closure: (String, Int) -> Void) {
    closure("Key", 7)
}

// This should not throw a compiler error because the use is not ambiguous.
performAction { value in
    print(value)
}

// Workaround
performAction { (value: Int) in
    print(value)
}

Seems like we're just not done removing the code that allowed a single closure argument to match a tuple.

(it does still reproduce in Swift 3)

@DougGregor, are there dups of this?

I don't know any off-hand; it's closely related to SE-0110, an implementation for which recently landed in master.

... but which didn't fix it.

Looks like this problem is fixed in Swift 4, mpdifran (JIRA User) Can you please verify and resolve this issue?

Comment by Mark DiFranco (JIRA)

Yup, tested with the attached playground. There's no longer an error.