deduction guides for std::valarray

template<typename T, std::size_t cnt>
valarray(const T(&)[cnt], std::size_t) -> valarray<T>;

This deduction guide is provided for std::valarray to allow deduction from array and size (note that deduction from pointer and size is covered by the implicit guides).

Example

#include <valarray>
int main() {
  int a[] = {1, 2, 3};
  std::valarray va(a, 3); // uses explicit deduction guide
}