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4386. std::simd::select(bool c, const T& a, const U& b) is underconstrained

Section: 29.10.8.13 [simd.alg] Status: New Submitter: Hewill Kang Opened: 2025-09-27 Last modified: 2025-10-22

Priority: 3

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Discussion:

This function currently only requires that c ? a : b be a well-formed expression, which simply returns c ? a : b.

Given that it seems intended to work with basic_vec, basic_mask, or vectorizable type, requiring T and U to be copyable seems reasonable since they are trivially copyable. It shouldn't take non-copyable objects and produce hard errors in the function body.

[2025-10-22; Reflector poll.]

Set priority to 3 after reflector poll.

Split opinion between further pinning down exact condition (is_constructible_v, decltype(auto(c ? a : b))), and accepting the PR (copyable) as this is not meant to be generic facility.

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.3 [simd.syn] as indicated:

   namespace std::simd {
     […]
     // 29.10.8.13 [simd.alg], algorithms
     […]
     template<copyableclass T, copyableclass U>
       constexpr auto select(bool c, const T& a, const U& b)
       -> remove_cvref_t<decltype(c ? a : b)>;
     […]
   };
   

2. Modify 29.10.8.13 [simd.alg] as indicated:

   template<copyableclass T, copyableclass U>
     constexpr auto select(bool c, const T& a, const U& b)
       -> remove_cvref_t<decltype(c ? a : b)>;
   

   -9- Effects: Equivalent to: return c ? a : b;