Should this code fail to compile in C++17?

Solution 1:

clang is correct here. Here's a reduced example:

struct B {
protected:
    B() { }
};

struct D : B { };

auto d = D{};

In C++14, D is not an aggregate because it has a base class, so D{} is "normal" (non-aggregate) initialization which invokes D's default constructor, which in turn invokes B's default constructor. This is fine, because D has access to B's default constructor.

In C++17, the definition of aggregate was widened - base classes are now allowed (as long as they're non-virtual). D is now an aggregate, which means that D{} is aggregate initialization. And in aggregate-initialization, this means that we (the caller) are initializing all the subobjects - including the base class subobject. But we do not have access to B's constructor (it is protected), so we cannot invoke it, so it is ill-formed.

Fear not, the fix is easy. Use parentheses:

auto d = D();

This goes back to invoking D's default constructor as before.