<list>, <string>: Remove allocator-moving tagged internal constructors

stl/inc/list

@@ -751,6 +751,10 @@ private:
 template <class _Traits>
 class _Hash;
 
+struct _Move_allocator_tag {
+    explicit _Move_allocator_tag() = default;
+};
+
 _EXPORT_STD template <class _Ty, class _Alloc = allocator<_Ty>>
 class list { // bidirectional linked list
 private:
@@ -804,8 +808,8 @@ public:
     }
 
 private:
-    template <class _Any_alloc>
-    explicit list(_Move_allocator_tag, _Any_alloc& _Al) : _Mypair(_One_then_variadic_args_t{}, _STD move(_Al)) {
+    template <class _Tag, class _Any_alloc, enable_if_t<is_same_v<_Tag, _Move_allocator_tag>, int> = 0>
+    explicit list(_Tag, _Any_alloc& _Al) : _Mypair(_One_then_variadic_args_t{}, _STD move(_Al)) {
         _Alloc_sentinel_and_proxy();
     }
 

stl/inc/sstream

@@ -239,7 +239,7 @@ public:
     // the buffer may already be full, and the terminating char is not '\0'.
     // In that case, copy the string as usual.
     _NODISCARD _Mystr str() && {
-        _Mystr _Result{_String_constructor_rvalue_allocator_tag{}, _STD move(_Al)};
+        _Mystr _Result{_Al};
         const auto _View = _Get_buffer_view();
         // _Size cannot be larger than _Res, but it could be equal,
         // because basic_stringbuf doesn't allocate for the terminating '\0'.

stl/inc/xmemory

@@ -1535,10 +1535,6 @@ public:
     }
 };
 
-struct _Move_allocator_tag {
-    explicit _Move_allocator_tag() = default;
-};
-
 template <class _Ty>
 pair<_Ty*, ptrdiff_t> _Get_temporary_buffer(ptrdiff_t _Count) noexcept {
     if (static_cast<size_t>(_Count) <= static_cast<size_t>(-1) / sizeof(_Ty)) {

stl/inc/xstring

@@ -508,11 +508,6 @@ struct _String_constructor_concat_tag {
     explicit _String_constructor_concat_tag() = default;
 };
 
-struct _String_constructor_rvalue_allocator_tag {
-    // tag to select constructors used by basic_stringbuf's rvalue str()
-    explicit _String_constructor_rvalue_allocator_tag() = default;
-};
-
 [[noreturn]] inline void _Xlen_string() {
     _Xlength_error("string too long");
 }
@@ -1145,12 +1140,6 @@ public:
 #endif // _HAS_CXX17
 
 #if _HAS_CXX20
-    basic_string(_String_constructor_rvalue_allocator_tag, _Alloc&& _Al)
-        : _Mypair(_One_then_variadic_args_t{}, _STD move(_Al)) {
-        // Used exclusively by basic_stringbuf
-        _Construct_empty();
-    }
-
     _NODISCARD bool _Move_assign_from_buffer(
         _Elem* const _Right, const size_type _Size, const size_type _Actual_allocation_size) {
         // Move assign from a buffer, used exclusively by basic_stringbuf; returns _Large_mode_engaged()

tests/std/test.lst

@@ -250,6 +250,7 @@ tests\GH_004618_mixed_operator_usage_keeps_statistical_properties
 tests\GH_004618_normal_distribution_avoids_resets
 tests\GH_004657_expected_constraints_permissive
 tests\GH_004845_logical_operator_traits_with_non_bool_constant
+tests\GH_004929_internal_tag_constructors
 tests\GH_004930_char_traits_user_specialization
 tests\LWG2381_num_get_floating_point
 tests\LWG2597_complex_branch_cut

tests/std/tests/GH_004929_internal_tag_constructors/env.lst

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

tests/std/tests/GH_004929_internal_tag_constructors/test.cpp

@@ -0,0 +1,93 @@
+// Copyright (c) Microsoft Corporation.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include <cassert>
+#include <list>
+#include <memory>
+#include <string>
+#include <type_traits>
+#if _HAS_CXX17
+#include <string_view>
+#endif // _HAS_CXX17
+
+#if _HAS_CXX20
+#define CONSTEXPR20 constexpr
+#else // ^^^ _HAS_CXX20 / !_HAS_CXX20 vvv
+#define CONSTEXPR20 inline
+#endif // ^^^ !_HAS_CXX20 ^^^
+
+using namespace std;
+
+template <class>
+constexpr bool is_initializer_list = false;
+template <class T>
+constexpr bool is_initializer_list<initializer_list<T>> = true;
+
+template <class T>
+constexpr initializer_list<T> ilist42 = {T{'4'}, T{'2'}};
+
+template <class>
+constexpr bool is_basic_string_or_cstr_or_view = false;
+template <class C, class T, class A>
+constexpr bool is_basic_string_or_cstr_or_view<basic_string<C, T, A>> = true;
+template <class C>
+constexpr bool is_basic_string_or_cstr_or_view<const C*> = true;
+#if _HAS_CXX17
+template <class C, class T>
+constexpr bool is_basic_string_or_cstr_or_view<basic_string_view<C, T>> = true;
+#endif // _HAS_CXX17
+
+struct nasty_string_source {
+    template <class IList, enable_if_t<is_initializer_list<IList>, int> = 0>
+    constexpr operator IList() const {
+        return ilist42<typename IList::value_type>;
+    }
+
+    template <class T, enable_if_t<!is_initializer_list<T> && !is_basic_string_or_cstr_or_view<T>, int> = 0>
+    constexpr operator T() const {
+        return T{};
+    }
+};
+
+CONSTEXPR20 bool test_nasty_conversion_to_basic_string() {
+    assert(string(nasty_string_source{}, allocator<char>{}) == "42"s);
+#ifdef __cpp_char8_t
+    assert(u8string(nasty_string_source{}, allocator<char8_t>{}) == u8"42"s);
+#endif // defined(__cpp_char8_t)
+    assert(u16string(nasty_string_source{}, allocator<char16_t>{}) == u"42"s);
+    assert(u32string(nasty_string_source{}, allocator<char32_t>{}) == U"42"s);
+    assert(wstring(nasty_string_source{}, allocator<wchar_t>{}) == L"42"s);
+
+    return true;
+}
+
+template <class>
+constexpr bool is_list = false;
+template <class T, class A>
+constexpr bool is_list<list<T, A>> = true;
+
+struct nasty_list_source {
+    template <class IList, enable_if_t<is_initializer_list<IList>, int> = 0>
+    constexpr operator IList() const {
+        return ilist42<typename IList::value_type>;
+    }
+
+    template <class T, enable_if_t<!is_initializer_list<T> && !is_list<T> && !is_integral_v<T>, int> = 0>
+    constexpr operator T() const {
+        return T{};
+    }
+};
+
+void test_nasty_conversion_to_list() {
+    allocator<int> ator{};
+    assert((list<int>{nasty_list_source{}, ator} == list<int>{int{'4'}, int{'2'}}));
+}
+
+#if _HAS_CXX20
+static_assert(test_nasty_conversion_to_basic_string());
+#endif // _HAS_CXX20
+
+int main() {
+    test_nasty_conversion_to_basic_string();
+    test_nasty_conversion_to_list();
+}