Explicitly specify additional template arguments - But I have no arguments left to specify

Question

The following code does not compile under clang (tested with version 10.0), but compiles under gcc (tested with version 10.1); C++ version 14. Which of the compilers is correct?

template< typename T > int func(); // (1)
template< typename ... Args > int func();  // (2)
template<> int func<int>() { return 1; }
int main {
  // func<int>();  // (*)
}

• The error message of clang is:

  function template specialization func ambiguously refers to more than one function template; explicitly specify additional template arguments to identify a particular function template

  My problem is, the function marked with (1) does not have additional template parameters, thus I cannot add additional template parameters. So, how to solve this problem?

• If I uncomment the line (*), then gcc also refuses to compile this code with the message:

  call of overloaded func<int>() is ambiguous.

Answer 1

The given program(even without the call) is not valid because you're trying to explicitly specialize function template func with information that is not enough to distinguish/disambiguate which func to specialize. A gcc bug report has been submitted here.

There are 2 ways to solve this depending on which func you want to specialize. Note also that the function call func<int>(); will still be ambiguous. The solution given below is just for specialization and not for call expression.

---

Method 1

My problem is, the function marked with (1) does not have additional template parameters, thus I cannot add additional template parameters. So, how to solve this problem?

In case, you want to explicitly specialize #1 you can solve the error by moving the explicit specialization above/before #2 as shown below:

template< typename T > int func(); // (1)
template<> int func<int>() { return 1; }  //OK NOW, explicitly specialize #1
template< typename ... Args > int func();  // (2)

int main() {
  
}

Working demo

Method 2

And in case you want to explicitly specialize #2 you should add more template argument in specialization as shown below:

template< typename T > int func(); // (1)

template< typename ... Args > int func();  // (2)
template<> int func<int, double>() { return 1; } //OK NOW, explicitly specialize #2
int main() {
  
}

Working demo