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4375. std::simd::bit_ceil should not be noexcept

Section: 29.10.8.15 [simd.bit] Status: Tentatively Ready Submitter: Matthias Kretz Opened: 2025-08-29 Last modified: 2025-10-22

Priority: Not Prioritized

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Discussion:

std::simd::bit_ceil is declared 'noexcept' in 29.10.3 [simd.syn] and 29.10.8.15 [simd.bit]. But

1. std::bit_ceil is not 'noexcept' (22.11.2 [bit.syn] and 22.11.5 [bit.pow.two]) and

2. std::simd::bit_ceil has a precondition.

[2025-10-22; Reflector poll.]

Set status to Tentatively Ready after six votes in favour during reflector poll.

Proposed resolution:

This wording is relative to N5014.

1. Modify 29.10.3 [simd.syn], header <simd> synopsis, as indicated:

   namespace std {
     […]
     // 29.10.8.15 [simd.bit], bit manipulation
     template<simd-vec-type V> constexpr V byteswap(const V& v) noexcept;
     template<simd-vec-type V> constexpr V bit_ceil(const V& v) noexcept;
     template<simd-vec-type V> constexpr V bit_floor(const V& v) noexcept;  
     […]
   }
   

2. Modify 29.10.8.15 [simd.bit] as indicated:

   template<simd-vec-type V> constexpr V bit_ceil(const V& v) noexcept;
   

   -3- Constraints: The type V::value_type is an unsigned integer type (6.9.2 [basic.fundamental]).

   -4- Preconditions: […]

   […]