C++: why constructor "A(A a) {}" is illegal? [duplicate]

Question

I met a quiz saying that the code below is ill-formed because "It is illegal to have a constructor whose first and only non-default argument is a value parameter for the class type."

I couldn't understand that. Why things like A(A a) : val (a.val) {} is ruled as illegal? why such a line in the standard? Is it because this will lead to ambiguity with copy constructor?

#include <iostream>

struct A
{
  A() : val() {}
  A(int v) : val(v) {}
  A(A a) : val(a.val) {} 

  int val;
};

int main(int argc, char** argv)
{
  A a1(5);
  A a2(a1);

  std::cout << a1.val + a2.val << std::endl;

  return 0;
}

Answer 1

A(A a) : val(a.val) {} will cause an infinite recursion because constructor arguments are being copied by value (invoking copy constructor and again the copy constructor and ...)