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From: "david.irvine at maidsafe dot net" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c++/56004] New: Possible bug with decltype and access modifer order
Date: Wed, 16 Jan 2013 15:13:00 -0000
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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56004

             Bug #: 56004
           Summary: Possible bug with decltype and access modifer order
    Classification: Unclassified
           Product: gcc
           Version: 4.7.2
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c++
        AssignedTo: unassigned@gcc.gnu.org
        ReportedBy: david.irvine@maidsafe.net


Please see this stackoverflow question for an overview.
http://stackoverflow.com/questions/14188535/clang-access-modifier-order-and-decltype

The issue seems to be that the private members are not visible at compilation
to the decltype call. This is a minimum example (from the question).  (I am the
questioner in this case). This also seems to appear as a 'bug' in gcc but not
msvc (12). I am not 100% convinced but cannot find in the standard why this
will not work. I hope this helps. 

#include <future>
#include <iostream>
#include <thread>
#include <vector>

template <class T> T &self(T &t) { return t;  }
template<typename T> struct Dependent {  };

template<typename T>
class Synchronised : Dependent<T>{
 public:
  explicit Synchronised(T t = T()) : t_(t) {}
  template<typename Functor>
  auto operator()(Functor functor) const ->decltype(functor(self(*this).t_)) {
  //auto operator()(Functor functor) const ->decltype(functor(this->t_)) {
    std::lock_guard<std::mutex> lock(mutex_);
    return functor(t_);
  }
 private:
  mutable T t_;
  mutable std::mutex mutex_;
};


int main() {

    Synchronised<std::string> sync_string("Start\n");
    std::vector<std::future<void>> futures;
}