Support lvalue references to incomplete types as `FunctionRef` parameters.

PiperOrigin-RevId: 514451518
Change-Id: Ic1391374c7bff08ba437ac0410631d1f701bb9ed

absl/functional/function_ref_test.cc

@@ -253,6 +253,16 @@ TEST(FunctionRef, PassByValueTypes) {
   }
   }
 }
 }
 
 
+TEST(FunctionRef, ReferenceToIncompleteType) {
+  struct IncompleteType;
+  auto test = [](IncompleteType&) {};
+  absl::FunctionRef<void(IncompleteType&)> ref(test);
+
+  struct IncompleteType {};
+  IncompleteType obj;
+  ref(obj);
+}
+
 }  // namespace
 }  // namespace
 ABSL_NAMESPACE_END
 ABSL_NAMESPACE_END
 }  // namespace absl
 }  // namespace absl

absl/functional/internal/function_ref.h

@@ -40,18 +40,21 @@ union VoidPtr {
 // Chooses the best type for passing T as an argument.
 // Chooses the best type for passing T as an argument.
 // Attempt to be close to SystemV AMD64 ABI. Objects with trivial copy ctor are
 // Attempt to be close to SystemV AMD64 ABI. Objects with trivial copy ctor are
 // passed by value.
 // passed by value.
+template <typename T,
+          bool IsLValueReference = std::is_lvalue_reference<T>::value>
+struct PassByValue : std::false_type {};
+
 template <typename T>
 template <typename T>
-constexpr bool PassByValue() {
-  return !std::is_lvalue_reference<T>::value &&
-         absl::is_trivially_copy_constructible<T>::value &&
-         absl::is_trivially_copy_assignable<
-             typename std::remove_cv<T>::type>::value &&
-         std::is_trivially_destructible<T>::value &&
-         sizeof(T) <= 2 * sizeof(void*);
-}
+struct PassByValue<T, /*IsLValueReference=*/false>
+    : std::integral_constant<bool,
+                             absl::is_trivially_copy_constructible<T>::value &&
+                                 absl::is_trivially_copy_assignable<
+                                     typename std::remove_cv<T>::type>::value &&
+                                 std::is_trivially_destructible<T>::value &&
+                                 sizeof(T) <= 2 * sizeof(void*)> {};
 
 
 template <typename T>
 template <typename T>
-struct ForwardT : std::conditional<PassByValue<T>(), T, T&&> {};
+struct ForwardT : std::conditional<PassByValue<T>::value, T, T&&> {};
 
 
 // An Invoker takes a pointer to the type-erased invokable object, followed by
 // An Invoker takes a pointer to the type-erased invokable object, followed by
 // the arguments that the invokable object expects.
 // the arguments that the invokable object expects.