<xmemory>: Tolerate bogus const-overloading in iterators passed to uninitialized_meow() (#5170)

Author: StephanTLavavej

stl/inc/xmemory

@@ -1602,7 +1602,7 @@ void _Return_temporary_buffer(_Ty* const _Pbuf) noexcept {
 }
 
 template <class _Ty, class _InIt>
-void _Construct_in_place_by_deref(_Ty& _Val, const _InIt& _Iter)
+void _Construct_in_place_by_deref(_Ty& _Val, _InIt& _Iter)
     noexcept(noexcept(::new (static_cast<void*>(_STD addressof(_Val))) _Ty(*_Iter))) {
     ::new (static_cast<void*>(_STD addressof(_Val))) _Ty(*_Iter);
 }
@@ -1632,14 +1632,14 @@ struct _NODISCARD _Uninitialized_backout {
     }
 
     template <class _InIt>
-    void _Emplace_back_deref(const _InIt& _Iter) {
+    void _Emplace_back_deref(_InIt& _Iter) {
         // construct a new element at *_Last from the result of dereferencing _Iter and increment.
         _STD _Construct_in_place_by_deref(*_Last, _Iter);
         ++_Last;
     }
 
     template <class _InIt>
-    void _Emplace_back_deref_move(const _InIt& _Iter) {
+    void _Emplace_back_deref_move(_InIt& _Iter) {
         // construct a new element at *_Last from the result of dereferencing _Iter and increment,
         // with lvalue cast to xvalue if necessary for uninitialized_move(_n).
         if constexpr (is_lvalue_reference_v<decltype(*_Iter)>) {

tests/std/test.lst

@@ -782,3 +782,4 @@ tests\VSO_1775715_user_defined_modules
 tests\VSO_1804139_static_analysis_warning_with_single_element_array
 tests\VSO_1925201_iter_traits
 tests\VSO_2252142_wrong_C5046
+tests\VSO_2318081_bogus_const_overloading

tests/std/tests/VSO_2318081_bogus_const_overloading/env.lst

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

tests/std/tests/VSO_2318081_bogus_const_overloading/test.compile.pass.cpp

@@ -0,0 +1,48 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cstddef>
+#include <iterator>
+#include <memory>
+using namespace std;
+
+// VSO-2318081 "[RWC][prod/fe][Regression] 4 projects failed with error C2440:
+// 'initializing': cannot convert from 'const llvm::Value *' to '_Ty'"
+
+struct Thing {};
+
+struct BogusInIt {
+    using iterator_category = input_iterator_tag;
+    using value_type        = Thing*;
+    using difference_type   = ptrdiff_t;
+    using pointer           = void;
+    using reference         = Thing*;
+
+    BogusInIt& operator++();
+    BogusInIt operator++(int);
+    friend bool operator==(const BogusInIt&, const BogusInIt&);
+    friend bool operator!=(const BogusInIt&, const BogusInIt&);
+
+    // This single overload would be conforming:
+    // Thing* operator*() const;
+
+    // N4993 [iterator.cpp17.general]/1 and [tab:inputiterator] forbid overloading operator*()
+    // with varying return types, but uninitialized_meow() tolerated this before GH-5135.
+
+    // See LLVM-119084, reported on 2024-12-07. After that has been fixed and propagated throughout the ecosystem,
+    // we should consider making the STL strictly reject such bogus iterators and removing this test coverage.
+    Thing* operator*();
+    const Thing* operator*() const;
+};
+
+void test() {
+    BogusInIt src{};
+    Thing** dest{nullptr};
+
+    uninitialized_copy(src, src, dest);
+    uninitialized_copy_n(src, 0, dest);
+#if _HAS_CXX17
+    uninitialized_move(src, src, dest);
+    uninitialized_move_n(src, 0, dest);
+#endif // _HAS_CXX17
+}