libstdc++: Handling user-defined interlike object in submdspan.

[Luc Grosheintz]

On 9/29/25 5:18 PM, Jonathan Wakely wrote:
>>
>> The question is should it support custom types that aren't copyable?
>> (Everything is much easier if all types are move and copy
>> constructable).
>>
> 
> I think this is in flux, via P3663, which is being reviewed by LWG at the
> moment. The latest draft (with a lot of input from Tomasz) is
> https://isocpp.org/files/papers/D3663R3.html
> 

Thank you, that's interesting. I'll read it again more carefully, but
it looks like the proposed casting layer is sensible; and might get
added via P3663 (they call it canonicalization). I'll finish a v1
with the casting layer.

>>
>> The idea I'm pursuing is to follow the pattern of casting slices to
>> "standard slices" immediately at the user-facing API layer, e.g.
>>
>>     template<typename _Slices...>
>>     constexpr auto
>>     submdspan_extents(_Slices... __slices)
>>     {
>>       return __submdspan_extents(__slice_cast(std::move(__slices))...);
>>     }
>>