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<flat_set>: Make flat_(multi)set insertion exception-safe for non-standard sequence containers #5972

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Merged
StephanTLavavej merged 2 commits into from
Jan 5, 2026
+29 −19
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<flat_set>: Make flat_(multi)set insertion exception-safe for non-standard sequence containers #5972

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538400d
Make `flat_(multi)set` insertion exception-safe for non-standard sequ…
frederick-vs-ja Dec 21, 2025
8b629c1
Update and move FIXME comment down to "erase".
StephanTLavavej Jan 5, 2026
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29 changes: 21 additions & 8 deletions stl/inc/flat_set
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Original file line number Diff line number Diff line change
Expand Up @@ -57,6 +57,18 @@ struct _NODISCARD _Flat_set_swap_clear_guard<true, _Container> {
void _Dismiss() noexcept {}
};

template <class _Container, class _Ty>
_NODISCARD auto _Emplace_with_clear_guard(_Container& _Cont, typename _Container::const_iterator _Where, _Ty&& _Val) {
if constexpr (_Has_guaranteed_single_insertion<_Container>) {
return _Cont.emplace(_Where, _STD forward<_Ty>(_Val));
} else {
_Clear_guard _Guard{_STD addressof(_Cont)};
auto _Iter = _Cont.emplace(_Where, _STD forward<_Ty>(_Val));
_Guard._Target = nullptr;
return _Iter;
}
}

_EXPORT_STD template <class _Kty, class _Keylt = less<_Kty>, class _Container = vector<_Kty>>
class flat_set;

Expand Down Expand Up @@ -279,8 +291,6 @@ public:
}

// modifiers
// FIXME, the "insert" and "erase" methods may not be able to restore the invariant, if the underlying
// container is unable to provide strong guarantee for "erase" and "insert" methods.
template <class... _Args>
auto emplace(_Args&&... _Vals) {
constexpr bool _Is_key_type = _In_place_key_extract_set<_Kty, remove_cvref_t<_Args>...>::_Extractable;
Expand Down Expand Up @@ -361,6 +371,9 @@ public:
_Guard._Target = nullptr;
}

// FIXME, the "erase" member functions need clear guards to restore the invariant when the underlying
// container doesn't provide the strong guarantee for its "erase" member functions.

// NB: `erase(iterator)` is identical to `erase(const_iterator)`
iterator erase(const_iterator _Where) {
return _Mycont.erase(_Where);
Expand Down Expand Up @@ -516,7 +529,7 @@ private:

if constexpr (is_same_v<remove_cvref_t<_Ty>, _Kty>) {
_STL_INTERNAL_CHECK(_Can_insert(_Where, _Val));
return pair{_Mycont.emplace(_Where, _STD forward<_Ty>(_Val)), true};
return pair{_STD _Emplace_with_clear_guard(_Mycont, _Where, _STD forward<_Ty>(_Val)), true};
} else {
// heterogeneous insertion
// FIXME: The standard only requires `find(_Val) == find(_Keyval)` (per N4958 [flat.set.modifiers]/2),
Expand All @@ -526,11 +539,11 @@ private:
_STL_ASSERT(_Can_insert(_Where, _Keyval),
"The conversion from the heterogeneous key to key_type should "
"be consistent with the heterogeneous lookup!");
return pair{_Mycont.emplace(_Where, _STD move(_Keyval)), true};
return pair{_STD _Emplace_with_clear_guard(_Mycont, _Where, _STD move(_Keyval)), true};
}
} else {
_STL_INTERNAL_STATIC_ASSERT(is_same_v<remove_cvref_t<_Ty>, _Kty>);
return _Mycont.emplace(upper_bound(_Val), _STD forward<_Ty>(_Val));
return _STD _Emplace_with_clear_guard(_Mycont, upper_bound(_Val), _STD forward<_Ty>(_Val));
}
}

Expand Down Expand Up @@ -560,7 +573,7 @@ private:

if constexpr (is_same_v<remove_cvref_t<_Ty>, _Kty>) {
_STL_INTERNAL_CHECK(_Can_insert(_Where, _Val));
return _Mycont.emplace(_Where, _STD forward<_Ty>(_Val));
return _STD _Emplace_with_clear_guard(_Mycont, _Where, _STD forward<_Ty>(_Val));
} else {
// heterogeneous insertion
// FIXME: The standard only requires `find(_Val) == find(_Keyval)` (per N4958 [flat.set.modifiers]/2),
Expand All @@ -570,7 +583,7 @@ private:
_STL_ASSERT(_Can_insert(_Where, _Keyval),
"The conversion from the heterogeneous key to key_type should "
"be consistent with the heterogeneous lookup!");
return _Mycont.emplace(_Where, _STD move(_Keyval));
return _STD _Emplace_with_clear_guard(_Mycont, _Where, _STD move(_Keyval));
}
} else {
_STL_INTERNAL_STATIC_ASSERT(is_same_v<remove_cvref_t<_Ty>, _Kty>);
Expand All @@ -591,7 +604,7 @@ private:
_Where = _STD lower_bound(_Where, _End, _Val, _Pass_comp());
}

return _Mycont.emplace(_Where, _STD forward<_Ty>(_Val));
return _STD _Emplace_with_clear_guard(_Mycont, _Where, _STD forward<_Ty>(_Val));
}
}

Expand Down
8 changes: 6 additions & 2 deletions stl/inc/xmemory
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Expand Up @@ -2762,10 +2762,14 @@ namespace ranges {
} // namespace ranges

template <class>
constexpr bool _Has_guaranteed_push_back = false; // N5014 [sequence.reqmts]/104, /108; used by flat_(multi)map::insert.
constexpr bool _Has_guaranteed_push_back = false; // N5032 [sequence.reqmts]/104, /108; used by flat_(multi)map::insert.

// N5032 [container.reqmts]/66.1; used by various flat_(multi)set inserting functions.
template <class _Ty>
constexpr bool _Has_guaranteed_single_insertion = _Has_guaranteed_push_back<_Ty>;

template <class>
constexpr bool _Has_guaranteed_append_range = false; // N5014 [sequence.reqmts]/112; used by flat_(multi)set::insert.
constexpr bool _Has_guaranteed_append_range = false; // N5032 [sequence.reqmts]/112; used by flat_(multi)set::insert.
#endif // _HAS_CXX23

template <class _Elem, class _UTy>
Expand Down
11 changes: 2 additions & 9 deletions tests/libcxx/expected_results.txt
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Expand Up @@ -51,15 +51,6 @@ std/strings/basic.string/string.modifiers/string_append/initializer_list.pass.cp
std/strings/basic.string/string.modifiers/string_assign/string.pass.cpp:0 FAIL
std/strings/basic.string/string.modifiers/string_assign/string.pass.cpp:1 FAIL

# LLVM-140448: [libc++][test] flat_set std test uses EmplaceUnsafeContainer which is non-standard
std/containers/container.adaptors/flat.set/flat.set.modifiers/emplace_hint.pass.cpp FAIL
std/containers/container.adaptors/flat.set/flat.set.modifiers/emplace.pass.cpp FAIL
std/containers/container.adaptors/flat.set/flat.set.modifiers/insert_cv.pass.cpp FAIL
std/containers/container.adaptors/flat.set/flat.set.modifiers/insert_iter_cv.pass.cpp FAIL
std/containers/container.adaptors/flat.set/flat.set.modifiers/insert_iter_rv.pass.cpp FAIL
std/containers/container.adaptors/flat.set/flat.set.modifiers/insert_rv.pass.cpp FAIL
std/containers/container.adaptors/flat.set/flat.set.modifiers/insert_transparent.pass.cpp FAIL

# LLVM-158302: Clang 20 i686-pc-windows-msvc regression, silent bad codegen for std::current_exception()
# SKIPPED because this is x86-specific.
std/language.support/support.exception/except.nested/assign.pass.cpp:2 SKIPPED
Expand Down Expand Up @@ -435,6 +426,8 @@ std/containers/container.adaptors/flat.set/flat.set.modifiers/insert_iter_iter.p
std/containers/container.adaptors/flat.set/flat.set.modifiers/insert_iter_iter.pass.cpp:1 FAIL
std/containers/container.adaptors/flat.set/flat.set.modifiers/insert_range.pass.cpp:0 FAIL
std/containers/container.adaptors/flat.set/flat.set.modifiers/insert_range.pass.cpp:1 FAIL
std/containers/container.adaptors/flat.set/flat.set.modifiers/insert_transparent.pass.cpp:0 FAIL
std/containers/container.adaptors/flat.set/flat.set.modifiers/insert_transparent.pass.cpp:1 FAIL
std/containers/container.adaptors/flat.set/flat.set.observers/comp.pass.cpp:0 FAIL
std/containers/container.adaptors/flat.set/flat.set.observers/comp.pass.cpp:1 FAIL
std/containers/container.adaptors/flat.set/flat.set.operations/contains_transparent.pass.cpp:0 FAIL
Expand Down