<ranges>: Strengthen exception specification for _Defaultabox

[frederick-vs-ja]

One non-defaulted move constructor for _Defaultabox is currently never noexcept, which is probably a bug. So I decided not to add // strengthened to that move constructor. Some similar constructors are made conditionally noexcept, and I think // strengthened is suitable for them.

_Defaultabox is used for join_view::_Iterator and lazy_split_view, while the standard wording uses optional and non-propagating-cache respectively. And thus I'm afraid that the exception specifications are not yet strong enough for lazy_split_view. But further strengthening seems dangerous.

[achabense]

_Defaultabox is not in standard. Do we need // strengthened for the noexcept-ness of its methods?

[StephanTLavavej]

@achabense is correct - we typically don't bother with "strengthened" comments for things that don't appear in the Standard. (For things that directly correspond to exposition-only functions, I think sometimes we do, but I forget specific examples.)

[achabense]

There are also a lot of /*strengthened*/s for other non-standard classes in this header 👀.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[StephanTLavavej]

🦾 ⚠️ 📦