[Bug c++/56152] New: explicit template instantiation of protected template function redeclared as public fails

[froydnj at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56152

             Bug #: 56152
           Summary: explicit template instantiation of protected template
                    function redeclared as public fails
    Classification: Unclassified
           Product: gcc
           Version: 4.4.7
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: froydnj@gcc.gnu.org


Compiling the following testcase:

enum Flavor {
  A,
  B
};

template<enum Flavor F>
class C
{
protected:
  template<typename T>
  static T f(T x)
  {
    if (F == A)
      return x;
    else
      return ~x;
  }
};

class AC : public C<A>
{
private:
  typedef C<A> super;

public:
  using super::f;
};

unsigned int
explicit_variable(unsigned int x)
{
  unsigned int (*func)(unsigned int) = AC::f<unsigned int>;

  return func(x);
}

unsigned int
explicit_call_instantiation(unsigned int x)
{
  return AC::f<unsigned int>(x);
}

unsigned int
implicit_call_instantiation(unsigned int x)
{
  return AC::f(x);
}

produces:

template-instantiation-bug.cpp: In function ‘unsigned int
explicit_variable(unsigned int)’:
template-instantiation-bug.cpp:11:12: error: ‘static T C<F>::f(T) [with T =
unsigned int, Flavor F = (Flavor)0u]’ is protected
template-instantiation-bug.cpp:32:44: error: within this context
template-instantiation-bug.cpp:11:12: error: ‘static T C<F>::f(T) [with T =
unsigned int, Flavor F = (Flavor)0u]’ is protected
template-instantiation-bug.cpp:32:44: error: within this context
template-instantiation-bug.cpp:11:12: error: ‘static T C<F>::f(T) [with T =
unsigned int, Flavor F = (Flavor)0u]’ is protected
template-instantiation-bug.cpp:32:44: error: within this context

It is not obvious to me that this is the right answer, given that both
explicit_call_instantiation and implicit_call_instantiation work.

All versions of GCC that I've tested reject the testcase (4.4 - 4.8).  Clang
(2.8 - 3.2) accepts the testcase.  MSVC version 9 rejects it.  My MSVC version
10 installation appears to be busted, or I'd test it there too.