Call Parameter as Reference to Array of Unknown Bound in C++

I am trying to understand whether references to array of unknown bound can be used as call parameter in functions in C++. Below is the example that i have:

EXAMPLE 1

void func(int (&a)[])
{
}
int main()
{
   cout << "Hello World" << endl; 
   int k[] = {1,2,3};
  // k[0] = 3;
   func(k);
   return 0;
}

To my surprise this example 1 above works when compiled with GCC 10.1.0 and C++11 but doesn't work with GCC version lower that 10.x. I don't think that we can have references to arrays of unknown size in C++. But then how does this code compile at the following link: successfully compiled

My second question is that can we do this for a function template? For example,

EXAMPLE 2

template<typename T1>
void foo(int (&x0)[])
{
}

Is example 2 valid C++ code in any version like C++17 etc. I saw usage of example 2 in a book where they have int (&x0)[] as a function parameter of a template function.