Proposed resolution:

This wording is relative to N5014.

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

   namespace std {
     […]
     // [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 [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 ([basic.fundamental]).

   -4- Preconditions: […]

   […]

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

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

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