Add Executor::KeepAlive copy-assignment operator

Summary:
[Folly] Add `Executor::KeepAlive` copy-assignment operator. Before this change, the copy-assignment operator is implicitly defined as deleted because it is not explicitly defined and `Executor::KeepAlive` has any of a user-declared move constructor or a user-declared move-assignment operator. This is true even though it has converting assignment operators that look like copy-assignment, because the copy-assignment operator is defined as deleted and it is more specific than the converting assignment operators so it is the best candidate overload at overload resolution time.

See: https://en.cppreference.com/w/cpp/language/copy_assignment.

Reviewed By: andriigrynenko

Differential Revision: D15836577

fbshipit-source-id: 4f0887875db2e0233d48d71f2f05d925300d3dc3

folly/Executor.h

@@ -104,6 +104,10 @@ class Executor {
       return *this;
     }
 
+    KeepAlive& operator=(KeepAlive const& other) {
+      return operator=(folly::copy(other));
+    }
+
     template <
         typename OtherExecutor,
         typename = typename std::enable_if<

folly/test/ExecutorTest.cpp

@@ -148,7 +148,8 @@ TEST(ExecutorTest, KeepAliveCopyAssignment) {
     EXPECT_EQ(&exec, ka.get());
     EXPECT_EQ(1, exec.refCount);
 
-    auto ka2 = ka;
+    decltype(ka) ka2;
+    ka2 = ka;
     EXPECT_TRUE(ka);
     EXPECT_TRUE(ka2);
     EXPECT_EQ(&exec, ka2.get());