Fix reference types in the return type

* Closes #35

Author: Naios

include/function2/function2.hpp

@@ -990,13 +990,13 @@ class vtable<property<IsThrowing, HasStrongExceptGuarantee, FormalArgs...>> {
 
   /// Invoke the function at the given index
   template <std::size_t Index, typename... Args>
-  constexpr auto invoke(Args&&... args) const {
+  constexpr decltype(auto) invoke(Args&&... args) const {
     auto thunk = invoke_table_t::template fetch<Index>(vtable_);
     return thunk(std::forward<Args>(args)...);
   }
   /// Invoke the function at the given index
   template <std::size_t Index, typename... Args>
-  constexpr auto invoke(Args&&... args) const volatile {
+  constexpr decltype(auto) invoke(Args&&... args) const volatile {
     auto thunk = invoke_table_t::template fetch<Index>(vtable_);
     return thunk(std::forward<Args>(args)...);
   }
@@ -1208,7 +1208,7 @@ class erasure : internal_capacity_holder<typename Config::capacity> {
   /// We define this out of class to be able to forward the qualified
   /// erasure correctly.
   template <std::size_t Index, typename Erasure, typename... Args>
-  static constexpr auto invoke(Erasure&& erasure, Args&&... args) {
+  static constexpr decltype(auto) invoke(Erasure&& erasure, Args&&... args) {
     auto const capacity = erasure.capacity();
     return erasure.vtable_.template invoke<Index>(
         std::forward<Erasure>(erasure).opaque_ptr(), capacity,
@@ -1324,7 +1324,7 @@ class erasure<false, Config,
   }
 
   template <std::size_t Index, typename Erasure, typename... T>
-  static constexpr auto invoke(Erasure&& erasure, T&&... args) {
+  static constexpr decltype(auto) invoke(Erasure&& erasure, T&&... args) {
     auto thunk = invoke_table_t::template fetch<Index>(erasure.invoke_table_);
     return thunk(&(erasure.view_), 0UL, std::forward<T>(args)...);
   }

test/regressions.cpp

@@ -198,3 +198,37 @@ TEST(regression_tests, unique_non_copyable) {
 
   ASSERT_EQ(view(), 5);
 }*/
+
+namespace issue_35 {
+class ref_obj {
+public:
+  ref_obj() = default;
+  ref_obj(ref_obj const&) = delete;
+  ref_obj(ref_obj&&) = default;
+  ref_obj& operator=(ref_obj&&) = default;
+  ref_obj& operator=(ref_obj const&) = delete;
+
+  int data() const {
+    return data_;
+  }
+
+private:
+  int data_{8373827};
+};
+
+ref_obj& ref_obj_getter() {
+  static ref_obj some;
+  return some;
+}
+} // namespace issue_35
+
+ALL_LEFT_TYPED_TEST_CASE(AllReferenceRetConstructTests)
+
+// https://github.com/Naios/function2/issues/35
+TYPED_TEST(AllReferenceRetConstructTests, reference_returns_not_buildable) {
+  using namespace issue_35;
+
+  typename TestFixture::template left_t<ref_obj&()> left(&ref_obj_getter);
+  ref_obj& ref = left();
+  ASSERT_EQ(ref.data(), 8373827);
+}