Why can't compiler deduce the return types?

Question

Can compiler not deduce the return type of a function if the return type happens to be a template parameter? Or I am making mistake in following code.

#include <iostream>

template <typename TT>                                                                                                                                                                                                                                                                                                                                                                        
TT retBoolFail(bool a) {                                                                                                                                                                                                                                                                                                                                                                      
    return true;                                                                                                                                                                                                                                                                                                                                                                              
}

template <typename TT>                                                                                                                                                                                                                                                                                                                                                                        
void retBoolSuccess(bool a, TT& ret) {                                                                                                                                                                                                                                                                                                                                                        
    ret = true;                                                                                                                                                                                                                                                                                                                                                                               
    return;                                                                                                                                                                                                                                                                                                                                                                                   
}

int main() {                                                                                                                                                                                                                                                                                                                                                                                  
    bool ret;                                                                                                                                                                                                                                                                                                                                                                                 
    retBoolSuccess(true, ret);  // Success                                                                                                                                                                                                                                                                                                                                                    
    retBoolFail(true);          // Failure                                                                                                                                                                                                                                                                                                                                                    
    return 0;                                                                                                                                                                                                                                                                                                                                                                                 
}

line 'RetBoolFail(true)' fails with following error.

-*- mode: compilation; default-directory: "~/work/c++/tupleTemplate/" -*-
Compilation started at Thu Mar 21 16:52:50

g++ -c simpletemp.cc  -std=c++1z -g; g++ -o simpletemp simpletemp.o
simpletemp.cc: In function ‘int main()’:
simpletemp.cc:17:21: error: no matching function for call to ‘retBoolFail(bool)’
     retBoolFail(true);          // Failure
                     ^
simpletemp.cc:4:4: note: candidate: template<class TT> TT retBoolFail(bool)
 TT retBoolFail(bool a) {
    ^
simpletemp.cc:4:4: note:   template argument deduction/substitution failed:
simpletemp.cc:17:21: note:   couldn't deduce template parameter ‘TT’
     retBoolFail(true);          // Failure
                     ^
Compilation finished at Thu Mar 21 16:52:50

Thanks.

Answer 1

No, deduction only occurs within the parameter list. You can use auto instead or just make it a non-template function with a hard-coded return type.