<memory>: Make algorithms properly destroy objects in constant evaluation (#5449)

frederick-vs-ja · web-flow · commit 53463dd8d0a5 · 2025-05-10T04:32:26.000-07:00
diff --git a/stl/inc/memory b/stl/inc/memory
@@ -584,12 +584,18 @@ namespace ranges {
     template <_No_throw_input_iterator _It, _No_throw_sentinel_for<_It> _Se>
         requires destructible<iter_value_t<_It>>
     _NODISCARD constexpr _It _Destroy_unchecked(_It _First, _Se _Last) noexcept {
-        if constexpr (is_trivially_destructible_v<iter_value_t<_It>>) {
-            _RANGES advance(_First, _STD move(_Last));
-        } else {
+        if (_STD is_constant_evaluated()) {
             for (; _First != _Last; ++_First) {
                 _RANGES destroy_at(_STD addressof(*_First));
             }
+        } else {
+            if constexpr (is_trivially_destructible_v<iter_value_t<_It>>) {
+                _RANGES advance(_First, _STD move(_Last));
+            } else {
+                for (; _First != _Last; ++_First) {
+                    _RANGES destroy_at(_STD addressof(*_First));
+                }
+            }
         }
 
         return _First;
@@ -630,12 +636,21 @@ _CONSTEXPR20 _NoThrowFwdIt destroy_n(_NoThrowFwdIt _First, const _Diff _Count_ra
     }
 
     auto _UFirst = _STD _Get_unwrapped_n(_First, _Count);
-    if constexpr (is_trivially_destructible_v<_Iter_value_t<_NoThrowFwdIt>>) {
-        _STD advance(_UFirst, _Count);
-    } else {
+#if _HAS_CXX20
+    if (_STD is_constant_evaluated()) {
         for (; _Count > 0; --_Count, (void) ++_UFirst) {
             _STD _Destroy_in_place(*_UFirst);
         }
+    } else
+#endif // _HAS_CXX20
+    {
+        if constexpr (is_trivially_destructible_v<_Iter_value_t<_NoThrowFwdIt>>) {
+            _STD advance(_UFirst, _Count);
+        } else {
+            for (; _Count > 0; --_Count, (void) ++_UFirst) {
+                _STD _Destroy_in_place(*_UFirst);
+            }
+        }
     }
 
     _STD _Seek_wrapped(_First, _UFirst);
@@ -658,14 +673,22 @@ namespace ranges {
             }
 
             auto _UFirst = _STD _Get_unwrapped_n(_STD move(_First), _Count);
-            if constexpr (is_trivially_destructible_v<iter_value_t<_It>>) {
-                _RANGES advance(_UFirst, _Count);
-            } else {
+            if (_STD is_constant_evaluated()) {
                 do {
                     _RANGES destroy_at(_STD addressof(*_UFirst));
                     ++_UFirst;
                     --_Count;
                 } while (_Count > 0);
+            } else {
+                if constexpr (is_trivially_destructible_v<iter_value_t<_It>>) {
+                    _RANGES advance(_UFirst, _Count);
+                } else {
+                    do {
+                        _RANGES destroy_at(_STD addressof(*_UFirst));
+                        ++_UFirst;
+                        --_Count;
+                    } while (_Count > 0);
+                }
             }
 
             _STD _Seek_wrapped(_First, _STD move(_UFirst));
diff --git a/stl/inc/xmemory b/stl/inc/xmemory
@@ -1111,11 +1111,19 @@ _CONSTEXPR20 void _Destroy_range(_Alloc_ptr_t<_Alloc> _First, const _Alloc_ptr_t
 
 template <class _NoThrowFwdIt, class _NoThrowSentinel>
 _CONSTEXPR20 void _Destroy_range(_NoThrowFwdIt _First, const _NoThrowSentinel _Last) noexcept {
-    // note that this is an optimization for debug mode codegen; in release mode the BE removes all of this
-    if constexpr (!is_trivially_destructible_v<_Iter_value_t<_NoThrowFwdIt>>) {
+#if _HAS_CXX20
+    if (_STD is_constant_evaluated()) {
         for (; _First != _Last; ++_First) {
             _STD _Destroy_in_place(*_First);
         }
+    } else
+#endif // _HAS_CXX20
+    { // note that this is an optimization for debug mode codegen; in release mode the BE removes all of this
+        if constexpr (!is_trivially_destructible_v<_Iter_value_t<_NoThrowFwdIt>>) {
+            for (; _First != _Last; ++_First) {
+                _STD _Destroy_in_place(*_First);
+            }
+        }
     }
 }
 
diff --git a/tests/std/tests/P0784R7_library_support_for_more_constexpr_containers/test.cpp b/tests/std/tests/P0784R7_library_support_for_more_constexpr_containers/test.cpp
@@ -583,6 +583,42 @@ static_assert(!CanDestroyN<const char*>);
 static_assert(!CanDestroyN<volatile char*>);
 static_assert(!CanDestroyN<const volatile char*>);
 
+#ifdef __clang__ // TRANSITION, DevCom-10642767 (MSVC), DevCom-10896316 (EDG)
+// Test that destroy, destroy_at, and destroy_n properly destroy trivially destructible objects
+// during constant evaluation.
+// After such destruction, further access will cause core language undefined behavior,
+// which will in turn cause constant evaluation failure.
+
+template <auto>
+struct require_valid_constant;
+
+template <class Fn>
+constexpr int consteval_validate_destruction(Fn op) {
+    struct S {
+        int n;
+    };
+
+    S arr[1]{{42}};
+    op(arr);
+    return arr[0].n;
+}
+
+template <auto Fn>
+constexpr bool CanWellDefinedlyAccessAfterOperation =
+    requires { typename require_valid_constant<consteval_validate_destruction(Fn)>; };
+
+static_assert(CanWellDefinedlyAccessAfterOperation<[](auto&) {}>);
+static_assert(!CanWellDefinedlyAccessAfterOperation<[](auto& arr) { destroy(arr + 0, arr + 1); }>);
+static_assert(!CanWellDefinedlyAccessAfterOperation<[](auto& arr) { destroy_at(arr + 0); }>);
+static_assert(!CanWellDefinedlyAccessAfterOperation<[](auto& arr) { destroy_at(&arr); }>);
+static_assert(!CanWellDefinedlyAccessAfterOperation<[](auto& arr) { destroy_n(arr + 0, 1); }>);
+static_assert(!CanWellDefinedlyAccessAfterOperation<[](auto& arr) { ranges::destroy(arr + 0, arr + 1); }>);
+static_assert(!CanWellDefinedlyAccessAfterOperation<[](auto& arr) { ranges::destroy(arr); }>);
+static_assert(!CanWellDefinedlyAccessAfterOperation<[](auto& arr) { ranges::destroy_at(arr + 0); }>);
+static_assert(!CanWellDefinedlyAccessAfterOperation<[](auto& arr) { ranges::destroy_at(&arr); }>);
+static_assert(!CanWellDefinedlyAccessAfterOperation<[](auto& arr) { ranges::destroy_n(arr + 0, 1); }>);
+#endif // ^^^ no workaround ^^^
+
 int main() {
     test_runtime(1234);
     test_runtime(string("hello world"));
