Allow Optionals to be compared to None

This is something like nullopt in std::experimental::optional. Optional
could already be constructed from None, so this seems like an obvious
extension from there.

I have a use in a future patch for Clang, though it may not go that
way/end up used - so this seemed worth committing now regardless.

llvm-svn: 245518

include/llvm/ADT/Optional.h

@@ -159,6 +159,25 @@ template <typename T> struct isPodLike<Optional<T> > {
 template<typename T, typename U>
 void operator==(const Optional<T> &X, const Optional<U> &Y);
 
+template<typename T>
+bool operator==(const Optional<T> &X, NoneType) {
+  return !X.hasValue();
+}
+
+template<typename T>
+bool operator==(NoneType, const Optional<T> &X) {
+  return X == None;
+}
+
+template<typename T>
+bool operator!=(const Optional<T> &X, NoneType) {
+  return !(X == None);
+}
+
+template<typename T>
+bool operator!=(NoneType, const Optional<T> &X) {
+  return X != None;
+}
 /// \brief Poison comparison between two \c Optional objects. Clients needs to
 /// explicitly compare the underlying values and account for empty \c Optional
 /// objects.

unittests/ADT/OptionalTest.cpp

@@ -377,5 +377,18 @@ TEST_F(OptionalTest, MoveGetValueOr) {
 
 #endif // LLVM_HAS_RVALUE_REFERENCE_THIS
 
+TEST_F(OptionalTest, NoneComparison) {
+  Optional<int> o;
+  EXPECT_EQ(o, None);
+  EXPECT_EQ(None, o);
+  EXPECT_FALSE(o != None);
+  EXPECT_FALSE(None != o);
+  o = 3;
+  EXPECT_FALSE(o == None);
+  EXPECT_FALSE(None == o);
+  EXPECT_TRUE(o != None);
+  EXPECT_TRUE(None != o);
+}
+
 } // end anonymous namespace