Implement ranges::ref_view

[miscco]

This implements ranges::ref_view which is a precursor for most other range adaptors.

NOTE: I need to come up with a reasonably simple test for the converting onstructor from a different range.

@CaseyCarter: Should we add empty to test::range?

[miscco]

Now i remember, it was actually not a squirrel but some three headed beast that let the poison pill fail to compile

[miscco]

Found the bug, it was sitting in front of the monitor...

[AdamBucior]

ref_view should not be in std::ranges::views

[miscco]

@AdamBucior thanks, I was confused by the range adaptor sentence that everything lives in std::ranges::views

[CaseyCarter]

@CaseyCarter: Should we add empty to test::range?

Why?

[miscco]

@CaseyCarter: Should we add empty to test::range?

Why?

because then I would not need to go with span to test it ;)

[CaseyCarter]

because then I would not need to go with span to test it ;)

I figured this out during my detailed review; see my comments inline which boil down to "write a concept to determine if ranges::empty(Read{}) is valid and ensure that ref_view<Read>::empty is valid iff that concept is satisfied."

[miscco]

FYI I am out until Monday, so if this is blocking you feel free to adopt whatever you need.

I will pick up the remnants later

[StephanTLavavej]

I observe that the requires says static_cast<_OtherRng&&>(_Other) instead of _STD forward<_OtherRng>(_Other). I am not sure if the variation is intentional. No change requested.

[CaseyCarter]

This is intentional: I try to avoid instantiating and/or specializing templates during overload resolution for throughput. In this case, that means static_cast<T&&>(t) in the constraints instead of std::forward<T>(t) that is used in noexcept-specifiers and function template bodies which are instantiated after overload resolution. There's an argument to be made that the difference is anomalous enough to offset any readability benefit.

[StephanTLavavej]

Thanks for implementing these ranges features so quickly! It might be a world record, we should call a ref over to view the instant replay. 🥇