1 files changed, 33 insertions, 3 deletions
diff --git a/absl/types/internal/optional.h b/absl/types/internal/optional.h
index 562c84ef..8acbda20 100644
--- a/absl/types/internal/optional.h
+++ b/absl/types/internal/optional.h
@@ -25,6 +25,34 @@
 #include "absl/meta/type_traits.h"
 #include "absl/utility/utility.h"
 
+// ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
+//
+// Inheriting constructors is supported in GCC 4.8+, Clang 3.3+ and MSVC 2015.
+// __cpp_inheriting_constructors is a predefined macro and a recommended way to
+// check for this language feature, but GCC doesn't support it until 5.0 and
+// Clang doesn't support it until 3.6.
+// Also, MSVC 2015 has a bug: it doesn't inherit the constexpr template
+// constructor. For example, the following code won't work on MSVC 2015 Update3:
+// struct Base {
+//   int t;
+//   template <typename T>
+//   constexpr Base(T t_) : t(t_) {}
+// };
+// struct Foo : Base {
+//   using Base::Base;
+// }
+// constexpr Foo foo(0);  // doesn't work on MSVC 2015
+#if defined(__clang__)
+#if __has_feature(cxx_inheriting_constructors)
+#define ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS 1
+#endif
+#elif (defined(__GNUC__) &&                                       \
+       (__GNUC__ > 4 || __GNUC__ == 4 && __GNUC_MINOR__ >= 8)) || \
+    (__cpp_inheriting_constructors >= 200802) ||                  \
+    (defined(_MSC_VER) && _MSC_VER >= 1910)
+#define ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS 1
+#endif
+
 namespace absl {
 
 // Forward declaration
@@ -108,7 +136,7 @@ template <typename T>
 class optional_data_base : public optional_data_dtor_base<T> {
  protected:
   using base = optional_data_dtor_base<T>;
-#if ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
+#ifdef ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
   using base::base;
 #else
   optional_data_base() = default;
@@ -151,7 +179,7 @@ class optional_data;
 template <typename T>
 class optional_data<T, true> : public optional_data_base<T> {
  protected:
-#if ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
+#ifdef ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
   using optional_data_base<T>::optional_data_base;
 #else
   optional_data() = default;
@@ -165,7 +193,7 @@ class optional_data<T, true> : public optional_data_base<T> {
 template <typename T>
 class optional_data<T, false> : public optional_data_base<T> {
  protected:
-#if ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
+#ifdef ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
   using optional_data_base<T>::optional_data_base;
 #else
   template <typename... Args>
@@ -361,4 +389,6 @@ struct optional_hash_base<T, decltype(std::hash<absl::remove_const_t<T> >()(
 }  // namespace optional_internal
 }  // namespace absl
 
+#undef ABSL_OPTIONAL_USE_INHERITING_CONSTRUCTORS
+
 #endif  // ABSL_TYPES_INTERNAL_OPTIONAL_H_