Fixed although the bug was voluntary so people know that Swift needs arguments labels...

You can work around the problem by adding empty arugemnt labels in your solution. Just change first line to:

func race(_ v1: Int, _ v2: Int, _ g: Int) -> [Int]? {

The function is not called correctly in Swift. I fixed the once for "Test" phase but they need to be fixed for the "Attempt" phase.

main.swift:14:23: error: missing argument labels 'v1:v2:g:' in call
XCTAssert(race(v1, v2, g)! == expected!, "should return (expected!)")
^
v1: v2: g:
main.swift:17:27: error: missing argument labels 'v1:v2:g:' in call
XCTAssertTrue(race(v1, v2, g) == nil, "Should return nil")
^
v1: v2: g: