When is a private constructor not a private constructor?

Question

Let's say I have a type and I want to make its default constructor private. I write the following:

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
}

Great.

But then, the constructor turns out to not be as private as I thought it was:

class C {
    C() = default;
};

int main() {
    C c{};         // OK on all compilers
    auto c2 = C{}; // OK on all compilers
}    

This strikes me as very surprising, unexpected, and explicitly undesired behavior. Why is this OK?

Answer 1

The trick is in C++14 8.4.2/5 [dcl.fct.def.default]:

... A function is user-provided if it is user-declared and not explicitly defaulted or deleted on its first declaration. ...

Which means that C's default constructor is actually not user-provided, because it was explicitly defaulted on its first declaration. As such, C has no user-provided constructors and is therefore an aggregate per 8.5.1/1 [dcl.init.aggr]:

An aggregate is an array or a class (Clause 9) with no user-provided constructors (12.1), no private or protected non-static data members (Clause 11), no base classes (Clause 10), and no virtual functions (10.3).