`<ranges>`: `operator|(_Left&& __l, _Right&& __r)` should return `decltype(auto)`

[hewillk]

STL/stl/inc/ranges

Lines 183 to 190 in bc5a5c3

	_EXPORT_STD template <class _Left, class _Right>
	requires (_Range_adaptor_closure_object<_Right> && range<_Left>)
	_NODISCARD constexpr auto operator|(_Left&& __l, _Right&& __r) noexcept(
	noexcept(_STD forward<_Right>(__r)(_STD forward<_Left>(__l))))
	requires requires { static_cast<_Right&&>(__r)(static_cast<_Left&&>(__l)); }
	{
	return _STD forward<_Right>(__r)(_STD forward<_Left>(__l));
	}

IIUC, LWG-3981 being tagged as NAD means it's a bug in MSVC-STL, right?

[StephanTLavavej]

We talked about this at the weekly maintainer meeting and we agree that this is a bug - WG21-N4964 [range.adaptor.object]/1

For a range adaptor closure object C and an expression R such that decltype((R)) models range, the following expressions are equivalent:
C(R)
R | C

does indeed appear to clearly mandate that decltype(auto) is used here.