a switch bug in swift? - "Switch must be exhaustive, consider adding a default clause."

Question

I am positive that the following swift code has covered all possibilities, but Xcode keeps telling me that, "Switch must be exhaustive, consider adding a default clause."

Can anyone tell me what did I miss? Thanks.

let a = false
let b = false
let c = false

func test(a: Bool, _ b: Bool, _ c: Bool) {
    switch (a, b, c) {
    case (true, false, _):
        print("Moved left!!!")
    case (true, true, _):
        print("Moved right!!!")
    case (false, _, false):
        print("Moved up!!!")
    case (false, _, true):
        print("Moved down!!!")
    // Error: Switch must be exhaustive, consider adding a default clause.
    }
}

test(false, false, false)
test(false, false, true)
test(false, true, false)
test(false, true, true)
test(true, false, false)
test(true, false, true)
test(true, true, false)
test(true, true, true)

Answer 1

The compiler is unable to conclude about your pattern because it is too complex or too unusual for it. If your pattern would have been more regular like:

case (true, false, _):
    print("Moved left!!!")
case (true, true, _):
    print("Moved right!!!")
case (false, false, _):
    print("Moved up!!!")
case (false, true, _):
    print("Moved down!!!")

then the compiler would have not complained. It that case it is easy for it to conclude that you covered all the cases.