Bug 56152 - explicit template instantiation of protected template function redeclared as public fails
Summary: explicit template instantiation of protected template function redeclared as ...
Status: NEW
Alias: None
Product: gcc
Classification: Unclassified
Component: c++ (show other bugs)
Version: 4.4.7
: P3 normal
Target Milestone: ---
Assignee: Not yet assigned to anyone
URL:
Keywords: rejects-valid
Depends on:
Blocks:
 
Reported: 2013-01-30 15:29 UTC by Nathan Froyd
Modified: 2021-12-12 11:40 UTC (History)
2 users (show)

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Host:
Target:
Build:
Known to work:
Known to fail:
Last reconfirmed: 2021-12-11 00:00:00

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Description Nathan Froyd 2013-01-30 15:29:15 UTC 
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.
Comment 1 Daniel Krügler 2013-01-30 20:24:06 UTC 
GCC 4.8.0 trunk behaves similar. I think the situation is currently not clear by the language. It looks like a manifestation of

http://www.open-std.org/jtc1/sc22/wg21/docs/cwg_active.html#1551

to me. Especially bullet 3 in the P/R looks relevant to me.
Comment 2 Andrew Pinski 2021-12-11 08:53:08 UTC 
Reduced testcase:
class C
{
protected:
  template<typename T>
  static T f(T x){return x;}
};

struct AC : C
{
  using C::f;
};

unsigned int (*func)(unsigned int) = AC::f;