Error:sorry, unimplemented: string literal in function template signature while Using decltype in function template

Question

Hi i am trying to understand how templates work and trying out different examples. I am getting an error while executing the code below. First i am writing the code and then will write what i think is happening.

Code snippet 1:

#include <iostream>
#include<vector>
#include<string>

template<typename It>
auto fcn(It beg, It end) -> decltype(*beg +"name"){ //why isn't decltype(*beg + "name") working here??
    
    return *beg;
}
int main()
{   
    std::vector<std::string> stringVector = {"firstElement","secondElement"};
    std::vector<std::string>::iterator beg = stringVector.begin();
    
    decltype(*beg + "k") l = "anothername";//this is okay and is working here.
    
    
    std::string s = fcn(stringVector.begin(), stringVector.end());
   
  
   return 0;
}

While compiling/executing snippet 1 i get an error.

sorry, unimplemented: string literal in function template signature auto fcn(It beg, It end) -> decltype(*beg +"name"){

Here is my explanation of what is happening: The return type of the function template fcn will be deduced from the expression decltype(*beg + "name"). So *beg is a reference to the element and in our case *beg is a reference to string that is string& and then *beg + "name" is a rvalue string. So decltype(*beg + "name") will give us a type string. But inside the definition of the function template fcn we are returning string& and therefore the return type of the function(string) and the type of the value returned from the function(string&) does not match and we get the error. But now as in the 2nd Code snippet i replace return *beg; with std::string temp = "name"; return temp; the return type and the type of the returned value should match. The problem is in the 2nd case i am still getting an error. Why am i getting this error and how can i resolve this? Also is my explanation of what is going on correct?

Code Snippet 2:

auto fcn(It beg, It end) -> decltype(*beg +"name"){ //why isn't decltype(*beg + "name") working here??
    std::string temp = "name";
    return temp;
}

Answer 1

This is GCC bug 47488, which has been resolved in GCC 9. Upgrade your compiler.

The function body is irrelevant. GCC simply couldn't deduce anything if the expression contained a string literal, so it gives up at decltype(*beg + "name"), which (like you correctly pointed out) should resolve simply to std::string when *beg is std::string&.