func foo(a: () -> Void) {}
func foo(b: () -> Void) {}
foo {}
// error: ambiguous use of 'foo' [ambiguous_decl_ref]
// foo {}
// ^
// note: use an explicit argument label instead of a trailing closure to call 'foo(a:)' [ambiguous_because_of_trailing_closure]
// foo {}
// ^  ~
//   (a:  )
// note: use an explicit argument label instead of a trailing closure to call 'foo(b:)' [ambiguous_because_of_trailing_closure]
// foo {}
// ^  ~
//   (b:  )

func foo(_ fn: () -> Void, a: () -> Void) {}
func foo(_ fn: () -> Void, b: () -> Void) {}
foo {} _: {}
//  error: no exact matches in call to global function 'foo' [no_overloads_match_exactly_in_call]
//  foo {} _: {}
//  ^
//  note: incorrect labels for candidate (have: '(_:_:)', expected: '(_:a:)') [candidate_expected_different_labels]
//  func foo(_ fn: () -> Void, a: () -> Void) {}
//       ^
//  note: incorrect labels for candidate (have: '(_:_:)', expected: '(_:b:)') [candidate_expected_different_labels]
//  func foo(_ fn: () -> Void, b: () -> Void) {}
//       ^