This does not compile.

broken.swift

class Foo {
    var closure: () -> () = {}

    init() {}
    
    func nothing() {}
    func setupClosure() {
        closure = {
            [weak self] in
            self?.nothing()   //  <-- this returns a "Void?", which means that the rhs's
                              //      inferred type does not match the lhs's type.
        }
    }
    
    func setupClosure2() {
        closure = {
            [weak self] () -> () in
            self?.nothing()   //  <-- Also fails to compile, because inferred type does not
                              //      match explicit type declaration
        }
    }
    
    func setupClosure3() {
        closure = {
            [weak self] in
            self?.nothing(); return  // <-- This works, because it's no longer a single-expression
                                     //     closure, and thus it's no longer inferring a "Void?" 
                                     //     return type.
        }
    }
}