<type_traits>: Workaround for common_reference

stl/inc/type_traits

@@ -1179,10 +1179,6 @@ struct basic_common_reference {};
 template <class _Ty>
 _Ty _Returns_exactly() noexcept; // not defined
 
-template <class _Ty1, class _Ty2>
-using _Cond_res = // N4810 [meta.trans.other]/2.4
-    decltype(false ? _Returns_exactly<_Ty1>() : _Returns_exactly<_Ty2>());
-
 template <class _From>
 struct _Copy_cv_impl {
     template <class _To>
@@ -1223,6 +1219,34 @@ struct _Add_qualifiers<_Ty1&&> {
     using _Apply = add_rvalue_reference_t<_Copy_cv<_Ty1, _Ty2>>;
 };
 
+#if !defined(__EDG__) && !defined(__clang__) // TRANSITION, DevCom-876860
+template <class _Ty1, class _Ty2>
+using _Cond_res_if_right = // N4810 [meta.trans.other]/2.4
+    decltype(false ? _Returns_exactly<_Ty1>() : _Returns_exactly<_Ty2>());
+
+template <class _Ty>
+using _Is_scalar_or_array = disjunction<is_scalar<_Ty>, is_array<_Ty>>;
+
+template <class _Ty1, class _Ty2, class = void>
+struct _Cond_res_workaround {};
+
+template <class _Ty1, class _Ty2>
+struct _Cond_res_workaround<_Ty1, _Ty2, void_t<_Cond_res_if_right<_Ty1, _Ty2>>> {
+    using _Uty = remove_cvref_t<_Ty1>;
+    using type = conditional_t<conjunction_v<is_same<_Uty, remove_cvref_t<_Ty2>>, _Is_scalar_or_array<_Uty>,
+                                   disjunction<conjunction<is_lvalue_reference<_Ty1>, is_rvalue_reference<_Ty2>>,
+                                       conjunction<is_rvalue_reference<_Ty1>, is_lvalue_reference<_Ty2>>>>,
+        decay_t<_Copy_cv<remove_reference_t<_Ty1>, remove_reference_t<_Ty2>>>, _Cond_res_if_right<_Ty1, _Ty2>>;
+};
+
+template <class _Ty1, class _Ty2>
+using _Cond_res = typename _Cond_res_workaround<_Ty1, _Ty2>::type;
+#else // ^^^ workaround / no workaround vvv
+template <class _Ty1, class _Ty2>
+using _Cond_res = // N4810 [meta.trans.other]/2.4
+    decltype(false ? _Returns_exactly<_Ty1>() : _Returns_exactly<_Ty2>());
+#endif
+
 template <class...>
 struct common_reference;
 

tests/libcxx/expected_results.txt

@@ -256,17 +256,6 @@ std/atomics/atomics.types.generic/copy_semantics_traits.pass.cpp FAIL
 # DevCom-409222 "Constructing rvalue reference from non-reference-related lvalue reference"
 std/utilities/meta/meta.unary/meta.unary.prop/is_constructible.pass.cpp:0 FAIL
 
-# DevCom-876860 "conditional operator errors" blocks readable<volatile int*>.
-std/concepts/concepts.lang/concept.common/common_with.compile.pass.cpp:0 FAIL
-std/concepts/concepts.lang/concept.commonref/common_reference.compile.pass.cpp:0 FAIL
-std/containers/views/span.cons/iterator_sentinel.pass.cpp:0 FAIL
-std/iterators/iterator.requirements/iterator.concepts/iterator.concept.bidir/bidirectional_iterator.compile.pass.cpp:0 FAIL
-std/iterators/iterator.requirements/iterator.concepts/iterator.concept.random.access/contiguous_iterator.compile.pass.cpp:0 FAIL
-std/iterators/iterator.requirements/iterator.concepts/iterator.concept.readable/indirectly_readable.compile.pass.cpp:0 FAIL
-std/iterators/iterator.requirements/iterator.concepts/iterator.concept.forward/forward_iterator.compile.pass.cpp:0 FAIL
-std/iterators/iterator.requirements/iterator.concepts/iterator.concept.input/input_iterator.compile.pass.cpp:0 FAIL
-std/iterators/iterator.requirements/iterator.concepts/iterator.concept.random.access/random_access_iterator.compile.pass.cpp:0 FAIL
-
 # VSO-1271673 "static analyzer doesn't know about short-circuiting"
 std/algorithms/alg.sorting/alg.sort/partial.sort/partial_sort.pass.cpp:0 FAIL
 std/algorithms/alg.sorting/alg.sort/partial.sort/partial_sort_comp.pass.cpp:0 FAIL

tests/libcxx/skipped_tests.txt

@@ -256,17 +256,6 @@ atomics\atomics.types.generic\copy_semantics_traits.pass.cpp
 # DevCom-409222 "Constructing rvalue reference from non-reference-related lvalue reference"
 utilities\meta\meta.unary\meta.unary.prop\is_constructible.pass.cpp
 
-# DevCom-876860 "conditional operator errors" blocks readable<volatile int*>.
-concepts\concepts.lang\concept.common\common_with.compile.pass.cpp
-concepts\concepts.lang\concept.commonref\common_reference.compile.pass.cpp
-containers\views\span.cons\iterator_sentinel.pass.cpp
-iterators\iterator.requirements\iterator.concepts\iterator.concept.bidir\bidirectional_iterator.compile.pass.cpp
-iterators\iterator.requirements\iterator.concepts\iterator.concept.random.access\contiguous_iterator.compile.pass.cpp
-iterators\iterator.requirements\iterator.concepts\iterator.concept.readable\indirectly_readable.compile.pass.cpp
-iterators\iterator.requirements\iterator.concepts\iterator.concept.forward\forward_iterator.compile.pass.cpp
-iterators\iterator.requirements\iterator.concepts\iterator.concept.input\input_iterator.compile.pass.cpp
-iterators\iterator.requirements\iterator.concepts\iterator.concept.random.access\random_access_iterator.compile.pass.cpp
-
 # VSO-1271673 "static analyzer doesn't know about short-circuiting"
 algorithms\alg.sorting\alg.sort\partial.sort\partial_sort.pass.cpp
 algorithms\alg.sorting\alg.sort\partial.sort\partial_sort_comp.pass.cpp

tests/std/tests/GH_002581_common_reference_workaround/env.lst

@@ -0,0 +1,4 @@
+# Copyright (c) Microsoft Corporation.
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+RUNALL_INCLUDE ..\strict_concepts_20_matrix.lst

tests/std/tests/GH_002581_common_reference_workaround/test.cpp

@@ -0,0 +1,42 @@
+#include <cstddef>
+#include <iterator>
+
+using namespace std;
+
+struct Test {};
+enum Unscoped {};
+enum class Scoped {};
+
+// Tests for raw pointers
+static_assert(contiguous_iterator<volatile int*>);
+static_assert(contiguous_iterator<const volatile int*>);
+static_assert(contiguous_iterator<volatile double*>);
+static_assert(contiguous_iterator<const volatile double*>);
+static_assert(contiguous_iterator<volatile nullptr_t*>);
+static_assert(contiguous_iterator<const volatile nullptr_t*>);
+static_assert(contiguous_iterator<volatile Unscoped*>);
+static_assert(contiguous_iterator<const volatile Unscoped*>);
+static_assert(contiguous_iterator<volatile Scoped*>);
+static_assert(contiguous_iterator<const volatile Scoped*>);
+static_assert(contiguous_iterator<Test* volatile*>);
+static_assert(contiguous_iterator<Test* const volatile*>);
+static_assert(contiguous_iterator<int Test::*volatile*>);
+static_assert(contiguous_iterator<int Test::*const volatile*>);
+
+// Tests for move_iterator specializations
+static_assert(input_iterator<move_iterator<volatile int*>>);
+static_assert(input_iterator<move_iterator<const volatile int*>>);
+static_assert(input_iterator<move_iterator<volatile double*>>);
+static_assert(input_iterator<move_iterator<const volatile double*>>);
+static_assert(input_iterator<move_iterator<volatile nullptr_t*>>);
+static_assert(input_iterator<move_iterator<const volatile nullptr_t*>>);
+static_assert(input_iterator<move_iterator<volatile Unscoped*>>);
+static_assert(input_iterator<move_iterator<const volatile Unscoped*>>);
+static_assert(input_iterator<move_iterator<volatile Scoped*>>);
+static_assert(input_iterator<move_iterator<const volatile Scoped*>>);
+static_assert(input_iterator<move_iterator<Test* volatile*>>);
+static_assert(input_iterator<move_iterator<Test* const volatile*>>);
+static_assert(input_iterator<move_iterator<int Test::*volatile*>>);
+static_assert(input_iterator<move_iterator<int Test::*const volatile*>>);
+
+int main() {} // COMPILE-ONLY