Bug 1669421 - Make andThen accept a function that accepts a rvalue reference. r=emilio

Differential Revision: https://phabricator.services.mozilla.com/D92552

mfbt/Result.h

@@ -737,8 +737,8 @@ class MOZ_MUST_USE_TYPE Result final {
    *     MOZ_ASSERT(res.unwrapErr() == res2.unwrapErr());
    */
   template <typename F, typename = std::enable_if_t<detail::IsResult<
-                            decltype((*((F*)nullptr))(*((V*)nullptr)))>::value>>
-  auto andThen(F f) -> decltype(f(*((V*)nullptr))) {
+                            std::invoke_result_t<F, V&&>>::value>>
+  auto andThen(F f) -> std::invoke_result_t<F, V&&> {
     return MOZ_LIKELY(isOk()) ? f(unwrap()) : propagateErr();
   }
 };

mfbt/tests/TestResult.cpp

@@ -532,20 +532,33 @@ static void OrElseTest() {
 
 static void AndThenTest() {
   // `andThen`ing over success results.
-  Result<int, const char*> r1(10);
-  Result<int, const char*> r2 =
-      r1.andThen([](int x) { return Result<int, const char*>(x + 1); });
-  MOZ_RELEASE_ASSERT(r2.isOk());
-  MOZ_RELEASE_ASSERT(r2.unwrap() == 11);
+  {
+    Result<int, const char*> r1(10);
+    Result<int, const char*> r2 =
+        r1.andThen([](int x) { return Result<int, const char*>(x + 1); });
+    MOZ_RELEASE_ASSERT(r2.isOk());
+    MOZ_RELEASE_ASSERT(r2.unwrap() == 11);
+  }
 
   // `andThen`ing over error results.
-  Result<int, const char*> r3("error");
-  Result<int, const char*> r4 = r3.andThen([](int x) {
-    MOZ_RELEASE_ASSERT(false);
-    return Result<int, const char*>(1);
-  });
-  MOZ_RELEASE_ASSERT(r4.isErr());
-  MOZ_RELEASE_ASSERT(r3.unwrapErr() == r4.unwrapErr());
+  {
+    Result<int, const char*> r3("error");
+    Result<int, const char*> r4 = r3.andThen([](int x) {
+      MOZ_RELEASE_ASSERT(false);
+      return Result<int, const char*>(1);
+    });
+    MOZ_RELEASE_ASSERT(r4.isErr());
+    MOZ_RELEASE_ASSERT(r3.unwrapErr() == r4.unwrapErr());
+  }
+
+  // andThen with a function accepting an rvalue
+  {
+    Result<int, const char*> r1(10);
+    Result<int, const char*> r2 =
+        r1.andThen([](int&& x) { return Result<int, const char*>(x + 1); });
+    MOZ_RELEASE_ASSERT(r2.isOk());
+    MOZ_RELEASE_ASSERT(r2.unwrap() == 11);
+  }
 }
 
 using UniqueResult = Result<UniquePtr<int>, const char*>;