Allow trivially move constructible types to be relocatable (#1035)

Summary:
- If a type is not marked explicitly with `IsRelocatable`
  as `std::true_type`, we infer its relocability only based on the
  property of whether the type is trivially copyable.
- Extend the default relocability to also be true for trivially move
  constructible types.
Pull Request resolved: https://github.com/facebook/folly/pull/1035

Reviewed By: Orvid

Differential Revision: D14240127

Pulled By: yfeldblum

fbshipit-source-id: 1e15d312d1a8340417bba2beb1db30ce4c543b26

folly/Traits.h

@@ -456,10 +456,9 @@ template <class T>
 struct IsRelocatable : std::conditional<
                            traits_detail::has_IsRelocatable<T>::value,
                            traits_detail::has_true_IsRelocatable<T>,
-                           // TODO add this line (and some tests for it) when we
-                           // upgrade to gcc 4.7
-                           // std::is_trivially_move_constructible<T>::value ||
-                           is_trivially_copyable<T>>::type {};
+                           bool_constant<
+                               std::is_trivially_move_constructible<T>::value ||
+                               is_trivially_copyable<T>::value>>::type {};
 
 template <class T>
 struct IsZeroInitializable

folly/test/TraitsTest.cpp

@@ -21,6 +21,7 @@
 #include <type_traits>
 #include <utility>
 
+#include <folly/Portability.h>
 #include <folly/ScopeGuard.h>
 #include <folly/portability/GTest.h>
 
@@ -98,6 +99,21 @@ TEST(Traits, unset) {
   EXPECT_TRUE(IsRelocatable<F4>::value);
 }
 
+TEST(Traits, triviallyMoveConstructible) {
+  FOLLY_PUSH_WARNING
+  FOLLY_CLANG_DISABLE_WARNING("-Wunneeded-member-function")
+  struct TriviallyMoveConstructible {
+    TriviallyMoveConstructible(const TriviallyMoveConstructible&) = delete;
+    TriviallyMoveConstructible& operator=(const TriviallyMoveConstructible&) =
+        delete;
+    TriviallyMoveConstructible(TriviallyMoveConstructible&&) = default;
+    TriviallyMoveConstructible& operator=(TriviallyMoveConstructible&&) =
+        delete;
+  };
+  EXPECT_TRUE(IsRelocatable<TriviallyMoveConstructible>::value);
+  FOLLY_POP_WARNING
+}
+
 TEST(Traits, bitAndInit) {
   EXPECT_TRUE(IsZeroInitializable<int>::value);
   EXPECT_FALSE(IsZeroInitializable<vector<int>>::value);