[C++] Reconcile type promotion rules between if_else, case_when, coalesce, select

[asfimport]

These functions are all conceptually similar and need to promote argument types to a common type. Right now they all have different type promotion heuristics, and there are a few in-progress PRs to improve type support; once those land, we should also reconcile the type promotion logic they use - presumably, all of them should be able to use the exact same logic in the end.

Reporter: David Li / @lidavidm

Related issues:

• [C++] Support variable-width types in case_when function (relates to)
• [C++] Extend type support for if_else kernel (relates to)
• [C++] Improve type support for 'coalesce' kernel (relates to)
• [C++] Support dictionaries directly in case_when kernel (relates to)
• [C++] Include month-day-nano intervals in kernel type support (relates to)
• [C++] choose kernel missing support for list/struct types (is related to)
• [C++] replace_with_mask support missing for list/fixed_size_list/large_list/struct (is related to)
• [C++] Support dictionaries directly in coalesce kernel (is related to)
• [C++] Improve type support for 'replace_with_mask' kernel (is related to)

_{Note: This issue was originally created as ARROW-14105. Please see the migration documentation for further details.}

[asfimport]

David Li / @lidavidm:
In particular, one thing that needs improvement is dictionary handling. Right now the type promotion values don't get applied to indices/values; we should do things like reconcile dictionary<int8, utf8> and dictionary<int32, binary> to dictionary<int32, binary>. It may be worth making the type promotion helpers iterator-based so that we can try to treat dictionaries uniformly as well, instead of having to copy descriptors around (though this requires the implementation to be in the header, unfortunately).

[asfimport]

Todd Farmer / @toddfarmer:
This issue was last updated over 90 days ago, which may be an indication it is no longer being actively worked. To better reflect the current state, the issue is being unassigned. Please feel free to re-take assignment of the issue if it is being actively worked, or if you plan to start that work soon.