[Bug c++/82478] Rejects valid access to private member type that should be allowed by friend

[redi at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82478

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
Reduced:

struct true_type { static const bool value = true; };
struct false_type { static const bool value = false; };

template <typename T>
using void_t = void;

template <typename, typename = void>
struct HasPrivate
{ static const bool value = false; };

template <typename T>
struct HasPrivate<T, void_t<typename T::PRIVATE>>
{ static const bool value = true; };

class A {
    struct PRIVATE { };

    // Either should be sufficient.
    friend struct HasPrivate<A>;
    template <typename, typename> friend struct HasPrivate;
};

static_assert(HasPrivate<A>::value, "");


Clang compiles this, but GCC and EDG don't.

GCC says:


priv.cc:23:15: error: ‘struct A::PRIVATE’ is private within this context
 static_assert(HasPrivate<A>::value, "");
               ^~~~~~~~~~~~~
priv.cc:16:12: note: declared private here
     struct PRIVATE { };
            ^~~~~~~


EDG says:

"priv.cc", line 23: error: static assertion failed with ""
  static_assert(HasPrivate<A>::value, "");
  ^

1 error detected in the compilation of "priv.cc".


I don't know what the standard says about the effect of friendship on template
argument lists in a partial specialization.