<algorithm>: ranges::minmax accidentally constructed remove_cvref_t<iter_reference_t<I>>

[hewillk]

STL/stl/inc/algorithm

Lines 10108 to 10116 in 1c59a20

	using _Vty = range_value_t<_Rng>;
	// This initialization is correct, similar to the N4950 [dcl.init.aggr]/6 example
	minmax_result<_Vty> _Found = {static_cast<_Vty>(*_UFirst), _Found.min};
	if (_UFirst == _ULast) {
	return _Found;
	}
	while (++_UFirst != _ULast) { // process one or two elements
	auto _Prev = *_UFirst;

The last auto _Prev = *_UFirst; indicates that we constructed remove_cvref_t<iter_reference_t<I>> from iter_reference_t<I>, which is not guaranteed to be well-formed by the function's constraints.
I believe we should do _Vty _Prev(*_UFirst); instead.

Contrived testcase: https://godbolt.org/z/Gdb8qffdM

[StephanTLavavej]

Good catch - we talked about this at the weekly maintainer meeting and agree that this is a bug.

This also indicates that we have missing test coverage of ranges algorithms with this scenario.