Add const_iterable_sequence concept #154
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For a sequence S, `const_iterable_sequence<S>` means we can use `S const` as a sequence with the expected semantics. Specifically, we require that: * `S const` is a sequence * `S` and `S const` have the same cursor type and the same value type * `S` and `S const` have the same `const_element_type` * For every extended sequence concept (`multipass`, `bidirectional` etc), excluding `read_only_sequence`, both `S` and `S const` must satisfy the concept or both must not satisfy the concept; for example, if `S` is bidirectional then `S const` must be bidirectional and vice versa * If `S` models `read_only_sequence` then `S const` must model `read_only_sequence` -- that is, `S const` may strengthen the const-qualification of elements but not weaken it * Semantic requirement: iterating over `S const` yields "the same elements" as iterating over `S` (but let's gloss over exactly how we define "same"...) We've previously assumed all of these things are true (except possibly the `const_element_t` requirement), but it's good to have them checked by the type system where we can.
Apparently when using C++23 various headers have been moved around so we no longer get things automatically included.
As in several other places, std::tuple and std::pair need the C++23 `common_reference` changes so that we can do correct concept checking.
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## main #154 +/- ##
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For a sequence S,
const_iterable_sequence<S>means we can useS constas a sequence with the expected semantics.Specifically, we require that:
S constis a sequenceSandS consthave the same cursor type and the same value typeSandS consthave the sameconst_element_typemultipass,bidirectionaletc), excludingread_only_sequence, bothSandS constmust satisfy the concept or both must not satisfy the concept. For example, ifSis bidirectional thenS constmust be bidirectional and vice versaSmodelsread_only_sequencethenS constmust modelread_only_sequence-- that is,S constmay strengthen the const-qualification of elements but not weaken itS constyields "the same elements" as iterating overS(but let's gloss over exactly how we define "same"...)We've previously assumed all of these things are true (except possibly the
const_element_trequirement), but it's good to have them checked by the compiler where we can.