[Bug c++/52957] Missing suggestions on '=' and '==' confusion

["redi at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=52957

--- Comment #10 from Jonathan Wakely <redi at gcc dot gnu.org> 2012-04-14 13:16:18 UTC ---
(In reply to comment #9)
> (In reply to comment #8)
> Thanks for sharing this. This broadens my perception of the issues contributors
> have with GCC. That said, it should be possible right now to use a C++ wrapper
> around tree, and use that in the C++ FE (and require C++ to bootstrap the C++
> FE). What do you think about that?

I think that would help me, I don't know about others.

This is probably more suitable for the mailing list not bugzilla but it would
certainly help me if a tree is a FIELD_DECL then pass in a field_decl wrapper
around a tree, if it's a TREE_LIST pass in a tree_list wrapper around a tree.

Then functions/accessors that work on a TREE_LIST could be overloaded to work
on the wrapper, so you can use the original accessor on the raw tree if you
know what you're doing, or the type-safe overload on the wrapper which will
only compile if it's a valid operation.

But that wouldn't work when a function parameter is a tree that could be either
one type or another.  That would either require the function to be refactored
to take two parameters, or some other way to have a wrapper that could be more
than one thing e.g. make the wrapper a template parameterised on by an enum
bitmask


  template<tree_code C>
    struct tree_wrapper
    {
       tree t;

       explicit tree_wrapper(tree t) : t(t)
       { gcc_checking_assert ( code() & C ); }

       tree_code code() const { return TREE_CODE (t); }
    };

  typedef tree_wrapper<TREE_LIST> tree_list;
  typedef tree_wrapper<TARGET_EXPR> target_expr;
  typedef tree_wrapper<CONSTRUCTOR> constructor;
  // etc.

  void f( tree_list list );

  void g( target_expr expr );

  void h( tree_wrapper<TREE_LIST|TARGET_EXPR> tree )
  {
    if (tree.code() == TREE_LIST)
      f( tree_list(tree.t) );
    else
      g( target_expr(tree.t) );
  }


> Well, the wiki is just a minor example, but it is awfully slow, nobody has
> administrator login (I can access as Daniel Berlin and ban users, but little
> more), and it is a unsupported version with known security issues.

Ah yes, I see what you mean now.