Speculatively implement LWG-4270 Diagnose misuse of std::projected::operator*

stl/inc/xutility

@@ -866,8 +866,36 @@ namespace ranges {
         template <class _Ty>
         concept _Can_deref = requires(_Ty&& __t) { *static_cast<_Ty&&>(__t); };
 
+        struct _Must_have_ADL_found_iter_move_or_be_dereferenceable {};
+
         class _Cpo {
         private:
+            template <class _Ty>
+            _NODISCARD static auto _Choose_return_type() {
+                if constexpr (_Has_ADL<_Ty>) {
+                    return type_identity<decltype(iter_move(_STD declval<_Ty>()))>{}; // intentional ADL
+                } else if constexpr (_Can_deref<_Ty>) {
+                    using _Ref = decltype(*_STD declval<_Ty>());
+#if !defined(__clang__) && !defined(__EDG__) // TRANSITION, VSO-1008447, VSO-2066340
+                    if constexpr (is_function_v<remove_reference_t<_Ref>>) {
+                        return type_identity<_Ref&>{};
+                    } else
+#endif // ^^^ workaround ^^^
+                    {
+                        if constexpr (is_lvalue_reference_v<_Ref>) {
+                            return type_identity<remove_reference_t<_Ref>&&>{};
+                        } else {
+                            return type_identity<_Ref>{};
+                        }
+                    }
+                } else {
+                    return _Must_have_ADL_found_iter_move_or_be_dereferenceable{};
+                }
+            }
+
+            template <class _Ty>
+            using _Return_type = decltype(_Choose_return_type<_Ty>())::type;
+
             enum class _St { _None, _Custom, _Fallback };
 
             template <class _Ty>
@@ -886,8 +914,7 @@ namespace ranges {
 
         public:
             template <class _Ty>
-                requires (_Choice<_Ty>._Strategy != _St::_None)
-            _NODISCARD _STATIC_CALL_OPERATOR constexpr decltype(auto) operator()(_Ty&& _Val) _CONST_CALL_OPERATOR
+            _NODISCARD _STATIC_CALL_OPERATOR constexpr _Return_type<_Ty> operator()(_Ty&& _Val) _CONST_CALL_OPERATOR
                 noexcept(_Choice<_Ty>._No_throw) {
                 constexpr _St _Strat = _Choice<_Ty>._Strategy;
 
@@ -1121,8 +1148,8 @@ struct _Projected_impl {
 
         using value_type = remove_cvref_t<indirect_result_t<_Proj&, _It>>;
         [[noreturn]] indirect_result_t<_Proj&, _It> operator*() const {
-            _STL_REPORT_ERROR("std::projected::operator*() can't be called (N5008 [projected]/1)");
-            _STL_UNREACHABLE; // no return value available for "continue on error"
+            static_assert(
+                false, "std::projected::operator*() can't be instantiated (N5008 [projected] as modified by LWG-4270)");
         }
     };
 };

tests/libcxx/expected_results.txt

@@ -1164,12 +1164,6 @@ std/containers/sequences/vector.bool/vector_bool.pass.cpp FAIL
 # Not analyzed. Inspecting shift operators for quoted().
 std/input.output/iostream.format/quoted.manip/quoted_traits.compile.pass.cpp FAIL
 
-# Not analyzed.
-# MSVC warning C5046: 'test_undefined_internal::A::operator *': Symbol involving type with internal linkage not defined
-# Clang error: function 'test_undefined_internal()::A::operator*' has internal linkage but is not defined [-Werror,-Wundefined-internal]
-std/iterators/iterator.requirements/iterator.cust/iterator.cust.move/iter_rvalue_reference_t.compile.pass.cpp FAIL
-std/iterators/predef.iterators/move.iterators/move.iter.ops/move.iter.op.comp/op_spaceship.pass.cpp FAIL
-
 # Not analyzed. Failing assert(arr[0].moves() == 1 && arr[1].moves() == 3).
 std/iterators/iterator.requirements/iterator.cust/iterator.cust.swap/iter_swap.pass.cpp FAIL
 
@@ -1518,9 +1512,6 @@ std/algorithms/alg.sorting/alg.set.operations/set.intersection/set_intersection_
 # Clang assertion: std::hermite(n, +inf) == inf
 std/numerics/c.math/hermite.pass.cpp FAIL
 
-# Not analyzed. Test coverage for LLVM-104496 uses span<Incomplete>.
-std/containers/views/views.span/span.cons/copy.pass.cpp FAIL
-
 # Not analyzed. These tests disable or limit allocations, which interferes with our proxy objects.
 std/algorithms/alg.sorting/alg.sort/stable.sort/stable_sort.pass.cpp FAIL
 std/containers/sequences/vector/vector.capacity/shrink_to_fit_exceptions.pass.cpp FAIL

tests/std/tests/P0896R4_ranges_iterator_machinery/test.cpp

@@ -1073,9 +1073,7 @@ namespace iterator_cust_move_test {
     static_assert(noexcept(ranges::iter_move(static_cast<int const*>(&some_ints[2]))));
 
     static_assert(same_as<iter_rvalue_reference_t<int[]>, int&&>);
-#if defined(__clang__) || defined(__EDG__) // TRANSITION, VSO-1008447, VSO-2066340
     static_assert(same_as<iter_rvalue_reference_t<int(int)>, int (&)(int)>);
-#endif // ^^^ no workaround ^^^
 
     static_assert(same_as<iter_rvalue_reference_t<int[4]>, int&&>);
     static_assert(ranges::iter_move(some_ints) == 0);
@@ -1084,9 +1082,7 @@ namespace iterator_cust_move_test {
     constexpr int f(int i) noexcept {
         return i + 1;
     }
-#if defined(__clang__) || defined(__EDG__) // TRANSITION, VSO-1008447
     static_assert(same_as<iter_rvalue_reference_t<int (*)(int)>, int (&)(int)>);
-#endif // ^^^ no workaround ^^^
     static_assert(ranges::iter_move(&f)(42) == 43);
     static_assert(noexcept(ranges::iter_move(&f)));