Proposed resolution:

This wording is relative to N5008.

1. Modify [string.find] as indicated:

   template<class T>
     constexpr size_type find(const T& t, size_type pos = 0) const noexcept(see below);
   […]
   template<class T>
     constexpr size_type find_last_not_of(const T& t, size_type pos = npos) const noexcept(see below);
   

   -2- Constraints: […]

   -3- Effects: Let G be the name of the function. Equivalent to:

   basic_string_view<charT, traits> s = *this, sv = t;
   return s.G(sv, pos);
   if (auto result = s.G(sv, pos); result == size_t(-1))
     return npos;
   else
     return result;
   

P1148R0 respecified the searching functions of `std::basic_string` to return corresponding string view type's `npos` member constant (equal to `std::size_t(-1)`), converted to the string type `S`'s member `S::size_type`, when the search fails. Before the change, `S::npos` (equal to `S::size_type(-1)`) was returned on failure.

On platforms where `std::size_t` isn't the widest unsigned integer type (e.g. on usual 32-bit platforms), the return value can change. Because there can be an allocator with a wider size_type, and when the basic_string type `S` uses such an allocator, `S::size_type` is specified to be that type, which in turn makes `S::size_type(std::size_t(-1))` not equal to `S::size_type(-1)`.

Do we want to restore the old return values?