func test(next: (inout () -> ()) -> ()) {
    var overwriteThisClosure = { print("bar") }
    next(&overwriteThisClosure)
    overwriteThisClosure()
}

test(next: {
    (foo: inout () -> ()) -> () in
    foo = { print("foo") }
}) // "foo"

func testGeneric<State>(next: (inout State) -> ()) {
    // ...
}

testGeneric(next: {
    (foo: inout () -> ()) -> () in
    foo = { print("foo") }
}) // "foo"

Command failed due to signal: Illegal instruction: 4
...
inout writeback in abstraction difference thunk not yet implemented
input value   %0 = argument of bb0 : $*@callee_owned (@in ()) -> @out ()
output type $*@callee_owned () -> ()

// dummy example (not even making use of the state 'foo') yielding the same compiler crash
var i = 0
let seq = sequence(
    state: { () -> () in () },
    next: { (foo: inout () -> ()) -> Int? in
        i += 1
        guard i < 5 else { return nil }
        return i
})