[Bug c++/16915] New: unnecessary copy construction

["boris at kolpackov dot net" <gcc-bugzilla at gcc dot gnu dot org>]

$ cat scope_lock.cxx

#include <iostream>

using std::cerr;
using std::endl;

struct lock_base
{
protected:
  lock_base ()
  {
  }

private:
  lock_base (lock_base const&);
};

template <typename T>
struct lock_ : lock_base
{
  lock_ (T&)
  {
    cerr << "locking" << endl;
  }

  lock_ (lock_ const&)
  {
    cerr << "copy" << endl;
  }

  ~lock_ ()
  {
    cerr << "unlocking" << endl;
  }
};

typedef lock_base const& lock;

struct mutex
{
  operator lock_<mutex> ()
  {
    cerr << "operator lock_<mutex>" << endl;
    return lock_<mutex> (*this);
  }
};

template <typename T>
lock_<T> flock (T& t)
{
  return lock_<T> (t);
}


int
main ()
{
  mutex m;
  
  {
    lock l (m);
    
    cerr << "end of block" << endl;
  }
}

$ g++-3.4 --version
g++-3.4 (GCC) 3.4.1 (Debian 3.4.1-5)

$ g++-3.4 scope_lock.cxx
scope_lock.cxx: In function `int main()':
scope_lock.cxx:20: error: `lock_base::lock_base(const lock_base&)' is private
scope_lock.cxx:67: error: within this context

Strangely, gcc tries to perform derived-to-base conversion and copy construction
when just derived-to-base conversion is already enough. The same code compiles
fine with icc 8.

-- 
           Summary: unnecessary copy construction
           Product: gcc
           Version: 3.4.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: c++
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: boris at kolpackov dot net
                CC: gcc-bugs at gcc dot gnu dot org
 GCC build triplet: i686-pc-linux-gnu
  GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=16915