Proposed resolution:

This wording is relative to N4901.

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

   […]
   template<class T>
     constexpr Tint bit_width(T x) noexcept;
   […]
   

2. Modify [bit.pow.two], as indicated:

   template<class T>
     constexpr Tint bit_width(T x) noexcept;
   

   -11- Constraints: T is an unsigned integer type ([basic.fundamental]).

   -12- Returns: If x == 0, 0; otherwise one plus the base-2 logarithm of x, with any fractional part discarded.