[PATCH, c++] Emit the diagnostic messages of convert_to_void

[Jason Merrill]

> +   The IMPLICIT is ICV_NULL when the user is explicitly converting an expression
> +   to void via a cast.

In that case let's call it ICV_CAST.

Let's improve the error messages here, too.

> +		    warning (0, "object of incomplete type %qT will not "
> +			    	"be accessed in void context", type);

"conversion to void will not access object of incomplete type %qT"

(Incidentally, I'm surprised these warnings are unconditional rather 
than being controlled by a -Wvolatile option or something, but I guess 
we can leave that alone for now since volatile is relatively rare.)

> +		    warning (0, "object of incomplete type %qT will not "
> +		    		"be accessed in second operand of conditional",

"indirection will not access object of incomplete type %qT in second 
operand of conditional expression" (or third)

"indirection will not access object of incomplete type %qT in right 
operand of comma operator" (or left)

"indirection will not access object of incomplete type %qT in statement"

"indirection will not access object of incomplete type %qT in for 
increment expression"

>  	/* Don't load the value if this is an implicit dereference, or if
>  	   the type needs to be handled by ctors/dtors.  */
>  	else if (is_volatile && (is_reference || TREE_ADDRESSABLE (type)))

Let's split up these cases.

is_reference:

"conversion to void will not access object of type %qT"

"implicit dereference will not access object of type %qT in..."

TREE_ADDRESSABLE:

"conversion to void will not access object of non-trivially-copyable 
type %qT"

"indirection will not access object of non-trivially-copyable type %qT 
in..."

> +		warning (0, "object %qE of incomplete type %qT will not "
> +			    "be accessed in void context", expr, type);

"conversion to void will not access object %qE of incomplete type %qT"

"variable %qE of incomplete type %qT will not be accessed in..."

> +		error ("void cast "
> +		       "cannot resolve address of overloaded function");

conversion to void, etc.

> +			 "second operand of conditional "
> +			 "is a reference, not call, to function %qE", expr);

conditional expression, etc.

> +		    warning (OPT_Wunused_value,
> +			     "second operand of conditional has no effect");

Likewise.

Jason