LWG-4499 `flat_set::insert_range` specification may be problematic

[StephanTLavavej]

LWG-4499 flat_set::insert_range specification may be problematic

[frederick-vs-ja]

We're now using append_range and insert_range and thus should have already avoided the problem shown in LWG-4499.

STL/stl/inc/flat_set

Lines 719 to 730 in fe3b0ac

	template <bool _NeedSorting, _Container_compatible_range<value_type> _Rng>
	void _Insert_range(_Rng&& _Range) {
	_Clear_guard _Guard{this};
	const size_type _Old_size = size();
	if constexpr (_Has_guaranteed_append_range<container_type>) {
	_Data.append_range(_STD forward<_Rng>(_Range));
	} else {
	_Data.insert_range(_Data.end(), _STD forward<_Rng>(_Range));
	}
	_Restore_invariants<_NeedSorting>(_Old_size);
	_Guard._Target = nullptr;
	}

Our current implementation strategy seems to be reasonable, but there's also possibly behavioral divergence from the standard wording.