GH-50720: [C++] Replace std::is_same_v with std::same_as

[dkp116]

Rationale for this change

Upgrade to meet c++20 standards, looking at implementation of same_as on cppref this can be replace is_same_v
https://en.cppreference.com/cpp/concepts/same_as

What changes are included in this PR?

Replace std::is_same_v with std::same_as

Are these changes tested?

Ran testing library, no need to create new test case

Are there any user-facing changes?

No

Was AI used for this PR?

No

In accordance to the AI generation guidelines, please disclose below whether and how AI was used in this PR.

PR code and description written by:

• [ X] Human
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Reviewed before submission by:

• [ X] Human
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• Not reviewed

• GitHub Issue: [C++] Replace std::is_same_v with std::same_as #50720

[taepper]

Did this change all usages of std::is_same_v in the codebase to std::same_as?

What is the rationale behind this? std::is_same_v is a type trait that evaluates to true or false, while std::same_as is a concept that is used define type constrains. Do you think it is advisable to e.g. replace this also in imperative code within template functions? If so, why?

[pitrou]

Same honest question: is this useful for maintenance, or is it just replacing a name with another?

(with the side effect of making the codebase history slightly more tedious to navigate)

[lidavidm]

Isn't this misunderstanding the point of the original issue? I would have thought the idea is to use the concept in some declarations (and then possibly be able to remove e.g. some static_asserts), not to just do a find-and-replace

[dkp116]

my mistake, misunderstood the issue