GH-48740: [C++] Add missing CTypeTraits for decimal types

[Naurder]

Rationale for this change

As reported in #48740, the CTypeTraits specializations were missing for decimal types. This prevented generic code from correctly mapping C++ decimal types to Arrow types.

What changes are included in this PR?

• Added CTypeTraits<Decimal128> mapping to Decimal128Type.
• Added CTypeTraits<Decimal256> mapping to Decimal256Type.

Are these changes tested?

Yes, via existing type traits tests in the CI pipeline.

Are there any user-facing changes?

No.

Closes #48740

• GitHub Issue: [C++] Some CTypeTraits are missing #48740

[Naurder]

Hi @HuaHuaY and @zanmato1984,
I picked up this issue and implemented the fix exactly as discussed in the closed PR #48763. Could you please take a look when you have a chance? Thanks!

[HuaHuaY]

I think it should inherit TypeTraits<Decimal128Type> like other specialization.

[Naurder]

Done. Thank you very much for suggestion.

[HuaHuaY]

There are some compile errors. Please fix them. The new CTypeTraits<Decimal128> instantiation should be placed after TypeTraits<Decimal128Type> instantiation.

[zanmato1984]

Thanks for updating this. The current version still needs changes: CI is now failing in the C++ build, and it looks caused by the placement of the new traits.

CTypeTraits<Decimal128> and CTypeTraits<Decimal256> are defined before TypeTraits<Decimal128Type> and TypeTraits<Decimal256Type>. Since the new CTypeTraits inherit from those TypeTraits, including arrow/type_traits.h instantiates the primary TypeTraits<T> first; the later explicit specializations are then rejected as “explicit specialization ... after instantiation”.

Could you move the decimal CTypeTraits definitions after the corresponding decimal TypeTraits definitions?

Also, the linked issue is about missing decimal CTypeTraits generally, and Decimal32/64 are still missing. Please add CTypeTraits<Decimal32> and CTypeTraits<Decimal64> as well.

Could you also add lightweight compile-time coverage, for example in type_test.cc, to check all four decimal mappings? Something along these lines should be enough:

static_assert(std::is_same_v<TypeTraits<Decimal32Type>::CType, Decimal32>);
static_assert(std::is_same_v<CTypeTraits<Decimal32>::ArrowType, Decimal32Type>);
// and similarly for Decimal64, Decimal128, Decimal256

[Naurder]

Moved the traits below their respective TypeTraits definitions, added the missing Decimal32/Decimal64, and included the compile-time coverage in type_test.cc.

[pitrou]

@Naurder Is this ready for another review? Also cc @zanmato1984 who did the first review already.

[Naurder]

@Naurder Is this ready for another review? Also cc @zanmato1984 who did the first review already.

Yes, the requested code changes are ready for another review.

But, CI is not green yet: 10 jobs are still failing, and I am currently
triaging the logs, starting with the C++ AVX2 job. I do not want to claim
that the PR is merge-ready until I understand those failures.

Thank you kindly for following up.

[Naurder]

@pitrou I've now rebased the branch onto the current main.

I did this because several of the previous CI failures looked unrelated to the decimal CTypeTraits change, and at least some of them were caused by issues that have already been fixed upstream since my last run. Rebasing should give us a cleaner signal from a fresh CI run against the current Arrow codebase.

The new CI run is currently waiting for maintainer approval (11 workflows). Once it is approved and finishes, I'll go through any remaining failures and address anything that is actually related to this PR.

Thanks again for taking another look.