GH-37148: [C++] Explicitly list the integer values of the Type::type enum

[felipecrv]

Rationale for this change

The enum type in C++ leaves the integer values implicitly defined and let the compiler assign the values to them automatically. This means an insertion of a new entry that is not at the end, causes implementations that rely on specific values (like R) to break with confusing error messages [1].

Assigning the values explicitly can communicate that these enum entry values are relied upon and can allow a more natural ordering of the list that is different from the numeric order the entries receive.

[1] #37091

What changes are included in this PR?

• Setting numeric values of the enum entries explicitly
• Completing an equivalent enum on the R side

Are these changes tested?

N/A

Are there any user-facing changes?

No.

• Closes: [C++] Implementation (like R) rely on implicit numeric assignment of enum Type entries #37148

[pitrou]

Would should we do about MAX_ID then?

[felipecrv]

Would should we do about MAX_ID then?

Leave it unset, so the compiler sets it to the maximum.

[thisisnic]

Thanks for fixing this! Just missing a comma on the line above this one, and then hopefully tests should pass.

[felipecrv]

Thank you! I fixed it and pushed again.

[thisisnic]

CI failures look to be a timeout and something with the dev script - I've re-triggered them both, but I think we can probably merge this once we're happy that's all they are?

[bkietz]

Explicit values for a large enumeration seems more readable to me, thanks!

[felipecrv]

@bkietz can you merge this?

[conbench-apache-arrow]

After merging your PR, Conbench analyzed the 6 benchmarking runs that have been run so far on merge-commit 8bdfc8c.

There were 9 benchmark results indicating a performance regression:

• Commit Run on ursa-i9-9960x at 2023-08-16 20:56:45Z
  • tpch (R) with engine=arrow, format=native, language=R, memory_map=False, query_id=TPCH-01, scale_factor=1
  • tpch (R) with engine=arrow, format=native, language=R, memory_map=False, query_id=TPCH-17, scale_factor=1
• and 7 more (see the report linked below)

The full Conbench report has more details. It also includes information about possible false positives for unstable benchmarks that are known to sometimes produce them.