is_constexpr_default_constructible

Summary: [Folly] `is_constexpr_default_constructible`, for deciding whether a type is constexpr default-constructible.

Reviewed By: vitaut

Differential Revision: D18092652

fbshipit-source-id: a4642726a9495135abca1506c5dbb3d57c0e0558

folly/Traits.h

@@ -168,6 +168,45 @@ constexpr bool is_instantiation_of_v = is_instantiation_of<C, T>::value;
 
 } // namespace detail
 
+namespace detail {
+
+template <bool, typename T>
+struct is_constexpr_default_constructible_;
+template <typename T>
+struct is_constexpr_default_constructible_<false, T> {
+  using type = std::false_type;
+};
+template <typename T>
+struct is_constexpr_default_constructible_<true, T> {
+  static constexpr int take(T) {
+    return 0;
+  }
+  template <int = take(T{})>
+  static std::true_type sfinae(int);
+  static std::false_type sfinae(...);
+  using type = decltype(sfinae(0));
+};
+
+} // namespace detail
+
+//  is_constexpr_default_constructible
+//  is_constexpr_default_constructible_v
+//
+//  A type trait, with associated variable template, which determines whether
+//  its type parameter is constexpr default-constructible, that is, default-
+//  constructible in a constexpr context.
+//
+//  Instantiations of is_constexpr_default_constructible unambiguously inherit
+//  std::integral_constant<bool, V> for some bool V.
+template <typename T>
+struct is_constexpr_default_constructible
+    : detail::is_constexpr_default_constructible_<
+          std::is_default_constructible<T>::value,
+          T>::type {};
+template <typename T>
+FOLLY_INLINE_VARIABLE constexpr bool is_constexpr_default_constructible_v =
+    is_constexpr_default_constructible<T>::value;
+
 /***
  *  _t
  *

folly/test/TraitsTest.cpp

@@ -401,3 +401,32 @@ TEST(Traits, is_instantiation_of) {
   EXPECT_TRUE((detail::is_instantiation_of_v<A, A<int>>));
   EXPECT_FALSE((detail::is_instantiation_of_v<A, B>));
 }
+
+TEST(Traits, is_constexpr_default_constructible) {
+  EXPECT_TRUE(is_constexpr_default_constructible_v<int>);
+
+  struct Empty {};
+  EXPECT_TRUE(is_constexpr_default_constructible_v<Empty>);
+
+  struct NonTrivialDtor {
+    ~NonTrivialDtor() {}
+  };
+  EXPECT_FALSE(is_constexpr_default_constructible_v<NonTrivialDtor>);
+
+  struct ConstexprCtor {
+    int x, y;
+    constexpr ConstexprCtor() noexcept : x(7), y(11) {}
+  };
+  EXPECT_TRUE(is_constexpr_default_constructible_v<ConstexprCtor>);
+
+  struct NonConstexprCtor {
+    int x, y;
+    NonConstexprCtor() noexcept : x(7), y(11) {}
+  };
+  EXPECT_FALSE(is_constexpr_default_constructible_v<NonConstexprCtor>);
+
+  struct NoDefaultCtor {
+    constexpr NoDefaultCtor(int, int) noexcept {}
+  };
+  EXPECT_FALSE(is_constexpr_default_constructible_v<NoDefaultCtor>);
+}