An odd error: "Escaping closure captures mutating 'self'"

[Jens]

@rayx

func test(_ callback: () -> Void) {  //  Not working. Must be @escaping
    let x: () -> Void = callback 
    x()
}

Removing the explicit type makes it compile:

func test(_ callback: () -> Void) { // Compiles, no need for it to be @escaping
    let x = callback
    x()
}

Is this a compiler bug?