Proposed resolution:

This wording is relative to N4981.

1. Modify [global.functions] as indicated:

   […]

   -2- A call to a non-member function signature described in [support] through [thread] and [depr] shall behave as if the implementation declared no additional non-member function signatures. However, for each non-member function specified with at least one default argument, an implementation may declare additional function signatures with one or more trailing parameters that have default arguments removed.

   [Example 1: The following program is possibly ill-formed because of ambiguity in overload resolution.

   #include <cstddef>
   #include <string>
   
   // One overload of std::stoi ([string.conversions]) is specified as
   // int stoi(const string& str, size_t* idx = nullptr, int base = 10);
   // An implementation can declare these overloads instead of using default arguments:
   // int stoi(const string& str, size_t* idx, int base);
   // int stoi(const string& str, size_t* idx);
   // int stoi(const string& str);
   
   namespace usr {
     int stoi(const std::string&);
     int stoi(const std::string&, std::size_t*);
   }
   
   int main() {
     using std::stoi;
     using usr::stoi;
     int (*p1)(const std::string&) = stoi;                 // possibly ill-formed
     int (*p2)(const std::string&, std::size_t*) = stoi;   // possibly ill-formed
   }
   

   — end example]

Currently, std::from_chars and std::to_chars are specified with default arguments. Some implementors want to split them into more overloads to determine the base 10 at compile time (LLVM issue #91268). However, the current standard wording doesn't clearly allow such technique.