deducing references to const from rvalue arguments

Question

Okay, this may seem like a silly question, but here it goes:

template 
void foo(T& x)
{
}

int main()
{
    foo(42);
    // error in pa         


        

Answer 1

Be it template or normal functions, rvalue cannot be passed by reference. (so const T& works but not T&).

What is preventing C++ to instantiate the foo function template with T = const int instead?

Suppose, C++ allows and makes T = const int instead. Now after sometime you change foo as,

template
void foo (T& x)
{
  x = 0;
}

Now compiler has to generate error. For end user the experience will be strange, as for a valid statement like x = 0; it started giving error. That could be the reason that why compiler prevents at the first stage itself!