PATCH: "virtual copy constructor" breaks on some platforms

[Matthias Neeracher]

While testing with g++ 4.0 on an OS version that shall remain unnamed  
and that will be available to the general public in a few months, I  
suffered from consistent crashes in Score_engraver::clone(): It  
seemed that instead of clone_const_helper, this function called an  
entirely unrelated virtual function. It turns out that on this  
platform, g++ 4.0 currently has a bug with old style casts that cast  
away constness while casting to a non-leftmost base class.

I reported this bug to the appropriate authorities, but on close  
examination, it turns out that the entire hair-raising evil of the  
VIRTUAL_COPY_CONSTRUCTOR code is not necessary anyway:

 #define VIRTUAL_COPY_CONSTRUCTOR(Base, name)                   \
  /* Hack to fix constness: gcc >= 2.95 is correct in defining \
     typeof (*this) in a const member function to be const.   
*/        \
  virtual Base *clone_const_helper ()                          \
  {                                                            \
    return new name (*this);                                   \
  }                                                            \
  virtual Base *clone () const                                 \
  {                                                            \
    /* return new name (*this); */                             \
    Base *urg = (Base *) this;                                 \
    return urg->clone_const_helper ();                         \
    return new name (*this);                                   \
   }

This is evil because it

a) Uses an old style cast
b) Calls clone_const_helper in the Base class, which if that Base  
class is non-leftmost and the leftmost base class has a virtual copy  
constructor, will get the wrong instance of clone_const_helper. If  
done at all, the cast should have been done to "name *", not "Base *".

It's also completely unnecessary, because the only reason  
clone_const_helper was necessary was in conjunction with typeof, and  
typeof was eliminated from this code more than a year ago (Revision  
1.25).
Patch attached, works fine on the aforementioned platform with g++ 4.0.

Matthias

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