c++ more than one instance of overloaded function matches argument type

I've overloaded my function contains three times

// returns true if char c is contained in unordered map um bool contains(std::unordered_map<char, op>& um, char c){ return um.find(c) != um.end(); } // returns true if string s is contained in unordered map um bool contains(std::unordered_map<char, op>& um, std::string& s){ return s.length() == 1 && contains(um, s[0]); } // returns true if string s is contained in unordered map um bool contains(std::unordered_map<std::string, func>& um, std::string& s){ return um.find(s) != um.end(); } 

The parameters are different in each overloaded function. Yet, from the line (contains(opmap, q_front)) I get the error: more than one instance of overloaded function "contains" matches the argument list.

For reference, opmap is of type std::unordered_map<char, op>, and q_front is a string. op in this case is just struct I created- I can post if needed, but I feel it is unnceessary in this case.

My question is why I'm getting this error, as the function call above should uniquely call the second method header: bool contains(std::unordered_map<char, op>& um, std::string& s){ because of the type of opmap matches the first parameter, and the type of q_front is string.

UPDATE:

Full error message:

more than one instance of overloaded function "contains" matches the argument list: -- function "contains(std::unordered_map<char, op, std::hash<char>, std::equal_to<char>, std::allocator<std::pair<const char, op>>> &um, std::string s)" (declared at line 48 of "/Users/raleighclemens/Documents/Calc_cpp/calc.h") -- function "contains(std::unordered_map<char, op, std::hash<char>, std::equal_to<char>, std::allocator<std::pair<const char, op>>> &um, std::string &s)" (declared at line 49) -- argument types are: (std::unordered_map<char, op, std::hash<char>, std::equal_to<char>, std::allocator<std::pair<const char, op>>>, std::string)C/C++(308) 

MRE:

#include <iostream> #include <string> #include <functional> #include <unordered_map> #define LEFT 0 #define RIGHT 1 #define UNARY 0 #define BINARY 1 struct op{ char symbol; uint8_t precedence; uint8_t assoc; uint8_t type; std::function<double (double, double)> ashley; }; struct func{ std::string symbol; uint8_t type; std::function<double (double, double)> ashley; }; bool contains(std::unordered_map<char, op>& um, char c){ return um.find(c) != um.end(); } // returns true if string s is contained in unordered map um bool contains(std::unordered_map<char, op>& um, std::string& s){ return s.length() == 1 && contains(um, s[0]); } // returns true if string s is contained in unordered map um bool contains(std::unordered_map<std::string, func>& um, std::string& s){ return um.find(s) != um.end(); } int main(int argc, char** argv){ std::unordered_map<char, op> opmap; op op1{'+', 2, LEFT, BINARY, [=] (double a, double b){return a + b;}}; opmap.emplace('+', op1); std::cout << contains(opmap, "+");