2 files changed, 33 insertions, 0 deletions
diff --git a/absl/meta/type_traits.h b/absl/meta/type_traits.h
index c08e375..927244e 100644
--- a/absl/meta/type_traits.h
+++ b/absl/meta/type_traits.h
@@ -224,6 +224,23 @@ struct disjunction<> : std::false_type {};
 template <typename T>
 struct negation : std::integral_constant<bool, !T::value> {};
 
+// is_function()
+//
+// Determines whether the passed type `T` is a function type.
+//
+// This metafunction is designed to be a drop-in replacement for the C++11
+// `std::is_function()` metafunction for platforms that have incomplete C++11
+// support (such as libstdc++ 4.x).
+//
+// This metafunction works because appending `const` to a type does nothing to
+// function types and reference types (and forms a const-qualified type
+// otherwise).
+template <typename T>
+struct is_function
+    : std::integral_constant<
+          bool, !(std::is_reference<T>::value ||
+                  std::is_const<typename std::add_const<T>::type>::value)> {};
+
 // is_trivially_destructible()
 //
 // Determines whether the passed type `T` is trivially destructable.
diff --git a/absl/meta/type_traits_test.cc b/absl/meta/type_traits_test.cc
index 82b1be5..a7a9c5c 100644
--- a/absl/meta/type_traits_test.cc
+++ b/absl/meta/type_traits_test.cc
@@ -385,6 +385,22 @@ class Base {
 #define ABSL_GCC_BUG_TRIVIALLY_CONSTRUCTIBLE_ON_ARRAY_OF_NONTRIVIAL 1
 #endif
 
+TEST(TypeTraitsTest, TestIsFunction) {
+  struct Callable {
+    void operator()() {}
+  };
+  EXPECT_TRUE(absl::is_function<void()>::value);
+  EXPECT_TRUE(absl::is_function<void()&>::value);
+  EXPECT_TRUE(absl::is_function<void() const>::value);
+  EXPECT_TRUE(absl::is_function<void() noexcept>::value);
+  EXPECT_TRUE(absl::is_function<void(...) noexcept>::value);
+
+  EXPECT_FALSE(absl::is_function<void(*)()>::value);
+  EXPECT_FALSE(absl::is_function<void(&)()>::value);
+  EXPECT_FALSE(absl::is_function<int>::value);
+  EXPECT_FALSE(absl::is_function<Callable>::value);
+}
+
 TEST(TypeTraitsTest, TestTrivialDestructor) {
   // Verify that arithmetic types and pointers have trivial destructors.
   EXPECT_TRUE(absl::is_trivially_destructible<bool>::value);