[stdlib] Add other comparison operators to 0-ary tuples

Add `!=`, `<`, `<=`, `>`, `>=` to 0-ary tuples

Resolves: SR-4172 (https://bugs.swift.org/browse/SR-4172)

Author: Arjun Nayini

stdlib/public/core/Tuple.swift.gyb

@@ -23,14 +23,82 @@ comparableOperators = [
 
 }%
 
-/// Two tuples each of arity 0 are equal
+/// Returns a Boolean value indicating whether the corresponding components of
+/// two tuples are equal.
+///
+/// All arity zero tuples are equal.
+///
 /// - Parameters:
 ///   - lhs: An empty tuple.
 ///   - rhs: An empty tuple.
 public func ==(lhs: (), rhs: ()) -> Bool {
   return true
 }
 
+/// Returns a Boolean value indicating whether any corresponding components of
+/// the two tuples are not equal.
+///
+/// Two tuples each of arity zero are never unequal
+///
+/// - Parameters:
+///   - lhs: An empty tuple.
+///   - rhs: An empty tuple.
+public func !=(lhs: (), rhs: ()) -> Bool {
+    return false
+}
+
+/// Returns a Boolean value indicating whether the first tuple is ordered
+/// before the second in a lexicographical ordering.
+///
+/// An arity zero tuple is never strictly before another arity zero tuple in a
+/// lexicographical ordering
+///
+/// - Parameters:
+///   - lhs: An empty tuple.
+///   - rhs: An empty tuple.
+public func <(lhs: (), rhs: ()) -> Bool {
+    return false
+}
+
+/// Returns a Boolean value indicating whether the first tuple is ordered
+/// before or the same as the second in a lexicographical ordering.
+///
+/// An arity zero tuple is always before or the same as another arity zero tuple
+/// in a lexicographical ordering
+///
+/// - Parameters:
+///   - lhs: An empty tuple.
+///   - rhs: An empty tuple.
+public func <=(lhs: (), rhs: ()) -> Bool {
+    return true
+}
+
+/// Returns a Boolean value indicating whether the first tuple is ordered
+/// after the second in a lexicographical ordering.
+///
+/// An arity zero tuple is never strictly after another arity zero tuple in a
+/// lexicographical ordering
+///
+/// - Parameters:
+///   - lhs: An empty tuple.
+///   - rhs: An empty tuple.
+public func >(lhs: (), rhs: ()) -> Bool {
+    return false
+}
+
+/// Returns a Boolean value indicating whether the first tuple is ordered
+/// after or the same as the second in a lexicographical ordering.
+///
+/// An arity zero tuple is always after or the same as another arity zero tuple
+/// in a lexicographical ordering
+///
+/// - Parameters:
+///   - lhs: An empty tuple.
+///   - rhs: An empty tuple.
+public func >=(lhs: (), rhs: ()) -> Bool {
+    return true
+}
+
 % for arity in range(2,7):
 %   typeParams = [chr(ord("A") + i) for i in range(arity)]
 %   tupleT = "({})".format(",".join(typeParams))

test/stdlib/Tuple.swift.gyb

@@ -59,8 +59,13 @@ TupleTestSuite.test("Tuple/equality") {
 TupleTestSuite.test("Tuple/equality/sanity-check") {
   // sanity check all arities
 
-  // 0-ary tuples should be equal
   expectTrue(() == ())
+  expectFalse(() != ())
+  expectFalse(() < ())
+  expectTrue(() <= ())
+  expectFalse(() > ())
+  expectTrue(() >= ())
+
 % for arity in range(2, maxArity + 1):
 %   a = str(tuple(range(1, arity + 1)))
 %   b = "({}, 0)".format(", ".join([str(i) for i in range(1, arity)]))