Don't crash if a constant initializer refers to the constant.

From-SVN: r168162

gcc/go/gofrontend/expressions.cc

@@ -2528,7 +2528,9 @@ Const_expression::do_type()
   if (this->type_ != NULL)
     return this->type_;
 
-  if (this->seen_)
+  Named_constant* nc = this->constant_->const_value();
+
+  if (this->seen_ || nc->lowering())
     {
       this->report_error(_("constant refers to itself"));
       this->type_ = Type::make_error_type();
@@ -2537,7 +2539,6 @@ Const_expression::do_type()
 
   this->seen_ = true;
 
-  Named_constant* nc = this->constant_->const_value();
   Type* ret = nc->type();
 
   if (ret != NULL)

gcc/go/gofrontend/gogo.cc

@@ -1163,8 +1163,8 @@ Lower_parse_tree::constant(Named_object* no, bool)
 {
   Named_constant* nc = no->const_value();
 
-  // We can recursively a constant if the initializer expression
-  // manages to refer to itself.
+  // Don't get into trouble if the constant's initializer expression
+  // refers to the constant itself.
   if (nc->lowering())
     return TRAVERSE_CONTINUE;
   nc->set_lowering();