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