There are two things to be considered here.

First, T will be an unknown type in the place where we call 
which gives a compilation error.

Second, if we call like this:

std::string c = Add("SAM", "SUNG"); or,
char* c = Add("SAM", "SUNG"); 

the compiler will convey an error since the arguments to 
Add are interpretted as char pointers. The error may be 
something like, "cannot add two pointers....".