`<ranges>`: `ranges::to` use range-based for loops in append branches

[hewillk]

STL/stl/inc/__msvc_ranges_to.hpp

Lines 1124 to 1136 in 89ca073

	for (auto&& _Elem : _Range) {
	using _ElemTy = decltype(_Elem);
	if constexpr (_Can_emplace_back<_Container, _ElemTy>) {
	_Cont.emplace_back(_STD forward<_ElemTy>(_Elem));
	} else if constexpr (_Can_push_back<_Container, _ElemTy>) {
	_Cont.push_back(_STD forward<_ElemTy>(_Elem));
	} else if constexpr (_Can_emplace_end<_Container, _ElemTy>) {
	_Cont.emplace(_Cont.end(), _STD forward<_ElemTy>(_Elem));
	} else {
	_STL_INTERNAL_STATIC_ASSERT(_Can_insert_end<_Container, _ElemTy>);
	_Cont.insert(_Cont.end(), _STD forward<_ElemTy>(_Elem));
	}
	}

To save the include overhead of ranges::for_each, MSVC-STL uses range-based for loop to iterate over the range. However, this is not guaranteed to be well-formed as the former does not extract iterators and sentinels through ranges::begin/ranges::end.
A disgusting and contrived case could be:

https://godbolt.org/z/Kc3dsvnfW

#include <ranges>

struct Vector {
  void push_back(int);
};

struct OnlyADLRange {
  void begin() = delete;
  void end() = delete;
  friend int* begin(OnlyADLRange&);
  friend int* end(OnlyADLRange&);
};

int main() {
  std::ranges::contiguous_range auto r = OnlyADLRange{};
  auto v = r | std::ranges::to<Vector>(); // only well-formed in libstdc++
}

Above, only libstdc++ explicitly uses ranges::begin/ranges::end to get iterators-pair and iterates over the ranges via a while loop so there is no issue.