Objective-C++ : Fix ICE in potential_constant_expression_1.

We cannot, as things stand, handle Objective-C tree codes in
the switch and deal with this by calling out to a function that
has a dummy version when Objective-C is not enabled.

Because of the way the logic works (with a fall through to a
'sorry' in case of unhandled expressions), the function reports
cases that are known to be unsuitable for constant exprs. The
dummy function always reports 'false' and thus will fall through
to the 'sorry'.

gcc/c-family/ChangeLog:

	* c-objc.h (objc_non_constant_expr_p): New.
	* stub-objc.c (objc_non_constant_expr_p): New.

gcc/cp/ChangeLog:

	* constexpr.c (potential_constant_expression_1): Handle
	expressions known to be non-constant for Objective-C.

gcc/objc/ChangeLog:

	* objc-act.c (objc_non_constant_expr_p): New.

gcc/c-family/c-objc.h

@@ -101,6 +101,7 @@ extern void objc_add_synthesize_declaration (location_t, tree);
 extern void objc_add_dynamic_declaration (location_t, tree);
 extern const char * objc_maybe_printable_name (tree, int);
 extern bool objc_is_property_ref (tree);
+extern bool objc_non_constant_expr_p (tree);
 extern bool objc_string_ref_type_p (tree);
 extern void objc_check_format_arg (tree, tree);
 extern void objc_finish_function (void);

gcc/c-family/stub-objc.c

@@ -331,6 +331,12 @@ objc_is_property_ref (tree ARG_UNUSED (node))
   return 0;
 }
 
+bool
+objc_non_constant_expr_p (tree)
+{
+  return 0;
+}
+
 tree
 objc_maybe_build_component_ref (tree ARG_UNUSED (datum), tree ARG_UNUSED (component))
 {

gcc/cp/constexpr.c

@@ -8413,7 +8413,7 @@ potential_constant_expression_1 (tree t, bool want_rval, bool strict, bool now,
       return false;
 
     default:
-      if (objc_is_property_ref (t))
+      if (objc_non_constant_expr_p (t))
 	return false;
 
       sorry ("unexpected AST of kind %s", get_tree_code_name (TREE_CODE (t)));

gcc/objc/objc-act.c

@@ -1720,7 +1720,6 @@ objc_build_class_component_ref (tree class_name, tree property_ident)
 }
 
 
-
 /* This is used because we don't want to expose PROPERTY_REF to the
    C/C++ frontends.  Maybe we should!  */
 bool
@@ -1732,6 +1731,21 @@ objc_is_property_ref (tree node)
     return false;
 }
 
+/* We use this to report tree codes that are known to be invalid in const-
+   expression contexts.  */
+bool
+objc_non_constant_expr_p (tree node)
+{
+  switch (TREE_CODE (node))
+    {
+      default:
+	return false;
+      case MESSAGE_SEND_EXPR:
+      case PROPERTY_REF:
+	return true;
+    }
+}
+
 /* This function builds a setter call for a PROPERTY_REF (real, for a
    declared property, or artificial, for a dot-syntax accessor which
    is not corresponding to a property).  'lhs' must be a PROPERTY_REF