std::experimental::simd_abi::scalar
From cppreference.com
                    
                                        
                    < cpp | experimental | simd
                    
                                                            
                    | Defined in header  <experimental/simd> | ||
| namespace simd_abi {   struct scalar {}; | ||
The scalar tag type ensures data-parallel types to store a single element (i.e., simd<T, simd_abi::scalar>::size() returns 1).
Notes
scalar is not an alias to fixed_size<1>.