The default parameter value given could not be converted to the type of the parameter. 

The message "Type mismatch in default argument value" is used when the parameter was not given a name. 

When compiling C++ programs, this message is always preceded by another message that explains the exact reason for the type mismatch. 

That other message is most often "Cannot convert 'type1' to 'type2'" but the mismatch could be due to another reason.