Revise is_instantiation_of_v and is_instantiation_of

Summary: [Folly] Revise is_instantiation_of_v as a variable and as eager, and is_instantiation_of as a type and as deferred.

Reviewed By: Mizuchi

Differential Revision: D23367989

fbshipit-source-id: bf6ebd74b8e7758818dfcebd04152e1297bc99e4

folly/Traits.h

@@ -147,20 +147,21 @@ using index_constant = std::integral_constant<std::size_t, I>;
 
 namespace detail {
 
-/**
- * A type trait to check if a given type is an instantiation of a class
- * template.
- *
- * Note that this only works with template type parameters. It does not work
- * with non-type template parameters, template template parameters, or alias
- * templates.
- */
+//  is_instantiation_of_v
+//  is_instantiation_of
+//
+//  A trait variable and type to check if a given type is an instantiation of a
+//  class template.
+//
+//  Note that this only works with type template parameters. It does not work
+//  with non-type template parameters, template template parameters, or alias
+//  templates.
 template <template <typename...> class, typename>
-struct is_instantiation_of : std::false_type {};
+FOLLY_INLINE_VARIABLE constexpr bool is_instantiation_of_v = false;
+template <template <typename...> class C, typename... T>
+FOLLY_INLINE_VARIABLE constexpr bool is_instantiation_of_v<C, C<T...>> = true;
 template <template <typename...> class C, typename... T>
-struct is_instantiation_of<C, C<T...>> : std::true_type {};
-template <template <typename...> class C, typename T>
-constexpr bool is_instantiation_of_v = is_instantiation_of<C, T>::value;
+struct is_instantiation_of : bool_constant<is_instantiation_of_v<C, T...>> {};
 
 } // namespace detail
 

folly/test/TraitsTest.cpp

@@ -418,11 +418,16 @@ TEST(Traits, like) {
            value));
 }
 
-TEST(Traits, is_instantiation_of) {
+TEST(Traits, is_instantiation_of_v) {
   EXPECT_TRUE((detail::is_instantiation_of_v<A, A<int>>));
   EXPECT_FALSE((detail::is_instantiation_of_v<A, B>));
 }
 
+TEST(Traits, is_instantiation_of) {
+  EXPECT_TRUE((detail::is_instantiation_of<A, A<int>>::value));
+  EXPECT_FALSE((detail::is_instantiation_of<A, B>::value));
+}
+
 TEST(Traits, is_constexpr_default_constructible) {
   constexpr auto const broken = kGnuc == 7 && !kIsClang;