instance Eq Unit

Summary: [Folly] instance Eq Unit

    Unit::operator==(const Unit&)
    Unit::operator!=(const Unit&)

Reviewed By: @Gownta

Differential Revision: D2179384

folly/futures/Unit.h

@@ -29,6 +29,8 @@ struct Unit {
   template <class T> struct Lift : public std::false_type {
     using type = T;
   };
+  bool operator==(const Unit& other) const { return true; }
+  bool operator!=(const Unit& other) const { return false; }
 };
 
 // Lift void into Unit.

folly/futures/test/UnitTest.cpp

@@ -26,6 +26,14 @@ TEST(Unit, futureDefaultCtor) {
   Future<Unit>();
 }
 
+TEST(Unit, operatorEq) {
+  EXPECT_TRUE(Unit{} == Unit{});
+}
+
+TEST(Unit, operatorNe) {
+  EXPECT_FALSE(Unit{} != Unit{});
+}
+
 TEST(Unit, voidOrUnit) {
   EXPECT_TRUE(is_void_or_unit<void>::value);
   EXPECT_TRUE(is_void_or_unit<Unit>::value);