<format>: Detect some invalid contiguous ranges in range_formatter

[frederick-vs-ja]

The following code snippet detects some "impossible" huge range.

STL/stl/inc/format

Lines 3283 to 3290 in 89ca073

	template <class _CharT, _RANGES input_range _Range, class _FormatContext>
	void _Range_formatter_format_as_string(_Range&& _Rng, _FormatContext& _Ctx, const bool _Debug) {
	if constexpr (_RANGES contiguous_range<_Range>) {
	const auto _Size = _STD _To_unsigned_like(_RANGES distance(_Rng));
	if (!_STD in_range<size_t>(_Size)) [[unlikely]] {
	_Throw_format_error("Formatted range is too long.");
	}

The semantic requirements of contiguous_iterator depend on pointer arithmetic, so when the underlying contiguous_range has more than 1 elements, the range must denote a contiguous element sequence of some array to be valid (otherwise, reaching from the beginning to the end causes UB). Moreover, the C++ object model doesn't permit an array object to have more than SIZE_MAX element. So we can infer that when the size of the contiguous range is too large to fit into size_t, the range must be invalid, and thus formatting it results in UB (per [iterator.requirements.general]/10, although the paragraph seemingly accidently makes infinite range invalid).

Fixes #5182.

[StephanTLavavej]

I haven't thought about this too deeply, but I'd recommend any pathological checks be controlled by _DEBUG or IDL=2, not CDL.