Allow const qualified FunctionRef instances. This allows the signature to be…

Allow const qualified FunctionRef instances. This allows the signature to be compatible with AnyInvokable for const uses.

PiperOrigin-RevId: 565682320
Change-Id: I924dadf110481e572bdb8af0111fa62d6f553d90

absl/functional/function_ref.h

@@ -137,6 +137,14 @@ class FunctionRef<R(Args...)> {
   absl::functional_internal::Invoker<R, Args...> invoker_;
 };
 
+// Allow const qualified function signatures. Since FunctionRef requires
+// constness anyway we can just make this a no-op.
+template <typename R, typename... Args>
+class FunctionRef<R(Args...) const> : public FunctionRef<R(Args...)> {
+ public:
+  using FunctionRef<R(Args...)>::FunctionRef;
+};
+
 ABSL_NAMESPACE_END
 }  // namespace absl
 

absl/functional/function_ref_test.cc

@@ -47,6 +47,11 @@ TEST(FunctionRefTest, Function2) {
   EXPECT_EQ(1337, ref());
 }
 
+TEST(FunctionRefTest, ConstFunction) {
+  FunctionRef<int() const> ref(Function);
+  EXPECT_EQ(1337, ref());
+}
+
 int NoExceptFunction() noexcept { return 1337; }
 
 // TODO(jdennett): Add a test for noexcept member functions.