<flat_set>: Fix erase_if for flat_set/flat_multiset

stl/inc/flat_set

@@ -242,11 +242,9 @@ public:
     }
 
     _NODISCARD container_type extract() && {
-        // The container NEEDS to be cleared no matter what,
-        // which is not guaranteed by simply moving it away
-        // ("... valid but unspecified ...")
         container_type& _Cont = _Get_cont();
-        _Clear_scope_guard _Guard{this};
+        // always clears the container (N4950 [flat.set.modifiers]/14 and [flat.multiset.modifiers]/10)
+        _Clear_scope_guard<_Base_flat_set> _Guard{this};
         container_type _Temp = _STD move(_Cont);
         return _Temp;
     }
@@ -405,7 +403,7 @@ public:
     }
 
 private:
-    void inline _Assert_after_sorted_input() const {
+    void _Assert_after_sorted_input() const {
         _STL_ASSERT(_STD is_sorted(begin(), end(), _Get_comp()), "Input was not sorted!");
         if constexpr (!_Multi) {
             _STL_ASSERT(_Is_unique(), "Input was not unique!");
@@ -624,15 +622,21 @@ public:
 };
 
 _EXPORT_STD template <class _Kty, class _Keylt, class _Container, class _Pred>
-size_t erase_if(flat_set<_Kty, _Keylt, _Container>& _Val, _Pred _Predicate) {
-    _Clear_scope_guard _Guard{&_Val};
-    return _Erase_remove_if(_Val, _Pass_fn(_Predicate));
+_Container::size_type erase_if(flat_set<_Kty, _Keylt, _Container>& _Val, _Pred _Predicate) {
+    // clears the container to maintain the invariants when an exception is thrown (N4950 [flat.set.erasure]/5)
+    _Clear_scope_guard<flat_set<_Kty, _Keylt, _Container>> _Guard{_STD addressof(_Val)};
+    const auto _Erased_count = _Erase_remove_if(_Val, _Pass_fn(_Predicate));
+    _Guard._Clearable        = nullptr;
+    return _Erased_count;
 }
 
 _EXPORT_STD template <class _Kty, class _Keylt, class _Container, class _Pred>
-size_t erase_if(flat_multiset<_Kty, _Keylt, _Container>& _Val, _Pred _Predicate) {
-    _Clear_scope_guard _Guard{&_Val};
-    return _Erase_remove_if(_Val, _Pass_fn(_Predicate));
+_Container::size_type erase_if(flat_multiset<_Kty, _Keylt, _Container>& _Val, _Pred _Predicate) {
+    // clears the container to maintain the invariants when an exception is thrown (N4950 [flat.multiset.erasure]/5)
+    _Clear_scope_guard<flat_multiset<_Kty, _Keylt, _Container>> _Guard{_STD addressof(_Val)};
+    const auto _Erased_count = _Erase_remove_if(_Val, _Pass_fn(_Predicate));
+    _Guard._Clearable        = nullptr;
+    return _Erased_count;
 }
 
 template <class _Kty, class _Keylt, class _Container, class _Alloc>

tests/std/tests/P1222R4_flat_set/test.cpp

@@ -1,6 +1,7 @@
 // Copyright (c) Microsoft Corporation.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 
+#include <algorithm>
 #include <cassert>
 #include <deque>
 #include <flat_set>
@@ -190,6 +191,24 @@ void test_non_static_comparer() {
     assert_all_requirements_and_equals(a, {9, 7, 5, -1});
 }
 
+template <class C>
+void test_extract() {
+    constexpr int elements[]{1, 2, 3, 4};
+    C fs{1, 2, 3, 4};
+    auto cont = std::move(fs).extract();
+    assert(fs.empty());
+    assert(ranges::equal(cont, elements));
+}
+
+template <class C>
+void test_erase_if() {
+    constexpr int erased_result[]{1, 3};
+    C fs{1, 2, 3, 4};
+    erase_if(fs, [](int n) { return n % 2 == 0; });
+    assert(fs.size() == 2);
+    assert(ranges::equal(fs, erased_result));
+}
+
 int main() {
     test_spaceship_operator<flat_set<int>>();
     test_spaceship_operator<flat_multiset<int>>();
@@ -205,6 +224,12 @@ int main() {
 
     test_non_static_comparer();
 
+    test_extract<flat_set<int>>();
+    test_extract<flat_multiset<int>>();
+
+    test_erase_if<flat_set<int>>();
+    test_erase_if<flat_multiset<int>>();
+
     assert_basic<flat_set<int>>();
     assert_basic<flat_set<int, std::less<int>, deque<int>>>();