Re: [PR] GH-48590: [C++] Modernize type traits using C++20 concepts (Preliminary) [arrow]

["abhishek593 (via GitHub)" <gi...@apache.org>]

abhishek593 commented on PR #49348:
URL: https://github.com/apache/arrow/pull/49348#issuecomment-3936596863

   @pitrou I just want to post my analysis here.
   
   The code so far what I have submitted is standard-compliant (compiles on gcc-14, gcc-15, clang). But the check [C++ / C++ Minimal Build Example](https://github.com/apache/arrow/actions/runs/22235190347/job/64324893309?pr=49348) fails which uses gcc-13. This is because of a bug in gcc-13 when evaluating concept-constrained partial specialisation of nested templates. Hence, the error:
   
   
   Error from CI:
   
   ```
   /arrow/cpp/src/arrow/compute/kernels/codegen_internal.h:736:21: error: 'Arg0Value' has not been declared
   
     736 |           arg0, [&](Arg0Value v) { return builder.Append(functor.op.Call(ctx, v, &st)); },
   ```
   Code from codegen_internal.h(present in my changes):
   
   ```
   template <arrow_c_number_or_decimal Type>
   struct ArrayExec<Type> {
     static Status Exec(...) {
       // GCC 13 throws an error on `Arg0Value` and `OutValue` inside this lambda
       VisitArrayValuesInline<Arg0Type>(arg0, [&](Arg0Value v) {
         *out_data++ = functor.op.template Call<OutValue, Arg0Value>(ctx, v, &st);
       });
     }
   };
   ```
   
   Since CI uses these older compilers, we may have a few options if we want to proceed:
   
   1. Explicit qualification: Explicitly qualify the types wherever they are used inside the specialisation
   
   ```diff
   -   VisitArrayValuesInline<Arg0Type>(arg0, [&](Arg0Value v) {
   -      *out_data++ = functor.op.template Call<OutValue, Arg0Value>(ctx, v, &st);
   +   VisitArrayValuesInline<Arg0Type>(arg0, [&](typename ThisType::Arg0Value v) {
   +      *out_data++ = functor.op.template Call<typename ThisType::OutValue, typename ThisType::Arg0Value>(ctx, v, &st);
   ```
   
   2. Identify such instances and fallback to SFINAE (lot of work).
   3. Upgrade CI compilers.
   
   BTW, is there any reason why compilers are not being upgraded?


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