Fix up error in generic definition of folly::swap

Summary:
- Fix up wrong order of template arguments in Expected generic
  - Create tests to check up all 3 forms of swap for Expected

Reviewed By: ericniebler

Differential Revision: D9013316

fbshipit-source-id: d4cebd83b268a4c5c551ac970f67b598f117fa73

folly/Expected.h

@@ -1362,8 +1362,8 @@ inline bool operator>=(
 /**
  * swap Expected values
  */
-template <class Error, class Value>
-void swap(Expected<Error, Value>& lhs, Expected<Value, Error>& rhs) noexcept(
+template <class Value, class Error>
+void swap(Expected<Value, Error>& lhs, Expected<Value, Error>& rhs) noexcept(
     expected_detail::StrictAllOf<IsNothrowSwappable, Value, Error>::value) {
   lhs.swap(rhs);
 }

folly/test/ExpectedTest.cpp

@@ -297,11 +297,11 @@ TEST(Expected, Order) {
   EXPECT_EQ(vect, expected);
 }
 
-TEST(Expected, Swap) {
+TEST(Expected, SwapMethod) {
   Expected<std::string, E> a;
   Expected<std::string, E> b;
 
-  swap(a, b);
+  a.swap(b);
   EXPECT_FALSE(a.hasValue());
   EXPECT_FALSE(b.hasValue());
 
@@ -310,7 +310,7 @@ TEST(Expected, Swap) {
   EXPECT_FALSE(b.hasValue());
   EXPECT_EQ("hello", a.value());
 
-  swap(a, b);
+  b.swap(a);
   EXPECT_FALSE(a.hasValue());
   EXPECT_TRUE(b.hasValue());
   EXPECT_EQ("hello", b.value());
@@ -319,7 +319,63 @@ TEST(Expected, Swap) {
   EXPECT_TRUE(a.hasValue());
   EXPECT_EQ("bye", a.value());
 
-  swap(a, b);
+  a.swap(b);
+  EXPECT_EQ("hello", a.value());
+  EXPECT_EQ("bye", b.value());
+}
+
+TEST(Expected, StdSwapFunction) {
+  Expected<std::string, E> a;
+  Expected<std::string, E> b;
+
+  std::swap(a, b);
+  EXPECT_FALSE(a.hasValue());
+  EXPECT_FALSE(b.hasValue());
+
+  a = "greeting";
+  EXPECT_TRUE(a.hasValue());
+  EXPECT_FALSE(b.hasValue());
+  EXPECT_EQ("greeting", a.value());
+
+  std::swap(a, b);
+  EXPECT_FALSE(a.hasValue());
+  EXPECT_TRUE(b.hasValue());
+  EXPECT_EQ("greeting", b.value());
+
+  a = "goodbye";
+  EXPECT_TRUE(a.hasValue());
+  EXPECT_EQ("goodbye", a.value());
+
+  std::swap(a, b);
+  EXPECT_EQ("greeting", a.value());
+  EXPECT_EQ("goodbye", b.value());
+}
+
+TEST(Expected, FollySwapFunction) {
+  Expected<std::string, E> a;
+  Expected<std::string, E> b;
+
+  folly::swap(a, b);
+  EXPECT_FALSE(a.hasValue());
+  EXPECT_FALSE(b.hasValue());
+
+  a = "salute";
+  EXPECT_TRUE(a.hasValue());
+  EXPECT_FALSE(b.hasValue());
+  EXPECT_EQ("salute", a.value());
+
+  folly::swap(a, b);
+  EXPECT_FALSE(a.hasValue());
+  EXPECT_TRUE(b.hasValue());
+  EXPECT_EQ("salute", b.value());
+
+  a = "adieu";
+  EXPECT_TRUE(a.hasValue());
+  EXPECT_EQ("adieu", a.value());
+
+  folly::swap(a, b);
+  EXPECT_EQ("salute", a.value());
+  EXPECT_EQ("adieu", b.value());
 }
 
 TEST(Expected, Comparisons) {