[stdlib] 0-ary tuples should be equatable

Add an == method for comparing two empty tuples.

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

Author: Arjun Nayini

stdlib/public/core/Tuple.swift.gyb

@@ -23,6 +23,14 @@ comparableOperators = [
 
 }%
 
+/// Two tuples each of arity 0 are equal
+/// - 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

@@ -58,6 +58,9 @@ TupleTestSuite.test("Tuple/equality") {
 
 TupleTestSuite.test("Tuple/equality/sanity-check") {
   // sanity check all arities
+
+  // 0-ary tuples should be equal
+  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)]))