issues with class friendship and inheritance - c++

Question

I feel like I'm missing something obvious here, but I have a class that declares another class as a friend yet does not seem to have access to its private members.
I have minimized the issue as follows:
Widget.hpp:
template<typename T> class Foo;
template<typename T, typename U>
class WidgetBase
{
protected:
T* ptr;
public:
WidgetBase(T* ptr) : ptr{ptr} {}
virtual void f() = 0;
};
template<typename T>
class WidgetDerived : public WidgetBase<typename Foo<T>::Bar, T>
{
public:
using WidgetBase<typename Foo<T>::Bar, T>::WidgetBase;
using WidgetBase<typename Foo<T>::Bar, T>::ptr;
virtual void f() { ptr->x = 9; }
};
Foo.hpp:
#include "Widget.hpp"
template<typename T>
class Foo
{
private:
struct Bar
{
T x;
explicit Bar(T x) : x{x} {}
};
public:
Bar* bar;
explicit Foo(T x) : bar{new Bar{x}} {}
~Foo() { delete bar; }
WidgetDerived<T> foo_widget() { WidgetDerived<T> widget{bar}; return widget; }
friend class WidgetBase<Bar, T>;
friend class WidgetDerived<T>;
};
main.cpp:
#include "Foo.hpp"
int main()
{
Foo<int> foo(7);
auto widget = foo.foo_widget();
}
Using GCC 7.3.0, I attempt to compile with: g++ -o main main.cpp -pedantic -Wall -Werror -Wconversion -std=c++1z and receive the following error message:
In file included from Foo.hpp:1:0,
from main.cpp:1:
Widget.hpp: In instantiation of 'class WidgetDerived<int>':
main.cpp:6:34: required from here
Widget.hpp:14:7: error: 'struct Foo<int>::Bar' is private within this context
class WidgetDerived : public WidgetBase<typename Foo<T>::Bar, T>
^~~~~~~~~~~~~
In file included from main.cpp:1:0:
Foo.hpp:7:12: note: declared private here
struct Bar
^~~
Note that I declare both WidgetBase and WidgetDerived as friends of Foo. What am I missing here?

Answer 1

This is a GCC (G++) bug, see bug report. The code compiles just fine with clang 6.0.0. However, clang 5.0.0 also reject the code.
Unfortunately I can only suggest these two possible solutions. 1) make the struct public, or 2) compile with a recent version of clang.