Inheriting second-level base class constructor: silent error

Question

I ran into a very nasty error today, here is a MWE:

#include <iostream>

class X {

public: 
  X() { std::cout << "Default" << std::endl; }
  X(int a) { std::cout << a << std::endl; }
};

class Y : public X { };

class Z : public Y {
  using X::X;
};

int main() {
  Z instance{3};
}

Contrary to my expectations, "Default" gets printed. Admittedly, the code is faulty because the inherited constructors of Z try to initialize X without specifying how to construct Y(∗). But still, shouldn't the compiler complain? What's the rationale behind the default constructor of Y (and subsequently X) getting called, completely silently ignoring my parameter of 3? Is this documented somewhere in the standard? Or is it a bug in my compiler?

My environment is gcc version 6.2.1 20160916 (Red Hat 6.2.1-2). No compiler warning is produced even with -Weffc++ -Wall -Wextra -pedantic.

Answer 1

It's a g++ bug, the code is invalid. Only constructors from direct bases can be inherited:

[namespace.udecl] §3 If such a using-declaration names a constructor, the nested-name-specifier shall name a direct base class of the class being defined