Bug 18064 and covariant returns

[Nathan Sidwell]

We apparently have a gnu extension that allows a virtual function
returning void * to be overridden by one returning T *, provided
T is not a class type

search.c:
       else if (!pedantic
	       && can_convert (TREE_TYPE (base_type), TREE_TYPE (over_type)))
	/* GNU extension, allow trivial pointer conversions such as
	   converting to void *, or qualification conversion.  */
	{
	  /* can_convert will permit user defined conversion from a
	     (reference to) class type. We must reject them.  */
	  over_return = non_reference (TREE_TYPE (over_type));
	  if (CLASS_TYPE_P (over_return))
	    fail = 2;

This is not documented in extend.texi.

Do we want to keep the extension?

My preference would be to deprecate and remove it (it's easier to
remove than document), but I'm not passionate about that.

nathan
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