Proposed resolution:

This wording is relative to N5001.

1. Modify [template.bitset.general] as indicated:

   namespace std {
     template<size_t N> class bitset {
     public:
       // bit reference
       class reference {
       public:
         constexpr reference(const reference&) = default;
         constexpr ~reference();
         constexpr reference& operator=(bool x) noexcept;           // for b[i] = x;
         constexpr reference& operator=(const reference&) noexcept; // for b[i] = b[j];
         constexpr const reference& operator=(bool x) const noexcept;
         constexpr bool operator~() const noexcept;                 // flips the bit
         constexpr operator bool() const noexcept;                  // for x = b[i];
         constexpr reference& flip() noexcept;                      // for b[i].flip();
       };
       […]
     };
     […]
   }