private copy constructor/assignment operator and copy-initialization

Question

This is a followup of this question

in the following code, why does line 1 compiles while line 2 and 3 does not (using visual C++ 2010)

class ABase
{
protected:
    ABase() {}
    ~ABase() {}
private:
    ABase( const ABase& );
    const ABase& operator=( const ABase& );
};

class A : ABase
{
};

class B
{
public:
    B() {}
    ~B() {}
private:
    B( const B& );
    const B& operator=( const B& );
};

int main( void )
{
    A a = A(); // line 1
    A a2( a ); // line 2
    B b = B(); // line 3

    return 0;
}

(note BA is a copy of boost::noncopyable)

edit: My problem is not to know why line 2 and 3 does not compile (I know that, the copy constructor is private), but why line 1 does.

Answer 1

Indeed line 1 should not compile.

Apparently vc++2010 has problems enforcing language rules in this case (may be because they're related to a base class and not to the object itself).

g++ diagnostic message about line 1 is very clear

ncopy.cpp: In copy constructor ‘A::A(const A&)’:
ncopy.cpp:7: error: ‘ABase::ABase(const ABase&)’ is private
ncopy.cpp:12: error: within this context
ncopy.cpp: In function ‘int main()’:
ncopy.cpp:27: note: synthesized method ‘A::A(const A&)’ first required here 

Answer 2

The compiler is wrong in accepting the first use. Even if the copy is elided, the copy constructor must be accessible for the code to be correct.

In this particular case there is an implicitly declared copy constructor in A:

§12.8/4 If the class definition does not explicitly declare a copy constructor, one is declared implicitly.

That is implicitly defined:

§12.8/7 An implicitly-declared copy constructor is implicitly defined if it is used to initialize an object of its class type from a copy of an object of its class type or of a class type derived from its class type108). [Note: the copy constructor is implicitly defined even if the implementation elided its use (12.2). ] A program is ill-formed if the class for which a copy constructor is implicitly defined has:

— a nonstatic data member of class type (or array thereof) with an inaccessible or ambiguous copy constructor, or

— a base class with an inaccessible or ambiguous copy constructor.