title: When is a private constructor not a private constructor?
body: <p>Let's say I have a type and I want to make its default constructor private. I write the following:</p>
  
  <pre><code>class C {
      C() = default;
  };
  
  int main() {
      C c;           // error: C::C() is private within this context (g++)
                     // error: calling a private constructor of class 'C' (clang++)
                     // error C2248: 'C::C' cannot access private member declared in class 'C' (MSVC)
      auto c2 = C(); // error: as above
  }
  </code></pre>
  
  <p>Great.</p>
  
  <p>But then, the constructor turns out to not be as private as I thought it was:</p>
  
  <pre><code>class C {
      C() = default;
  };
  
  int main() {
      C c{};         // OK on all compilers
      auto c2 = C{}; // OK on all compilers
  }    
  </code></pre>
  
  <p>This strikes me as very surprising, unexpected, and explicitly undesired behavior. Why is this OK? </p>