Return nil if argument of NODE_RETURN and NODE_NEXT is not specified.

src/codegen.c

@@ -1571,15 +1571,20 @@ codegen(codegen_scope *s, node *tree, int val)
     break;
 
   case NODE_RETURN:
+    if (tree) {
       codegen(s, tree, VAL);
       pop();
+    }
+    else {
+      genop(s, MKOP_A(OP_LOADNIL, cursp()));
+    }
     if (s->loop) {
       genop(s, MKOP_AB(OP_RETURN, cursp(), OP_R_RETURN));
     }
     else {
       genop_peep(s, MKOP_AB(OP_RETURN, cursp(), OP_R_NORMAL), NOVAL);
     }
-    push();
+    if (val) push();
     break;
 
   case NODE_YIELD:
@@ -1631,6 +1636,9 @@ codegen(codegen_scope *s, node *tree, int val)
         codegen(s, tree, VAL);
         pop();
       }
+      else {
+        genop(s, MKOP_A(OP_LOADNIL, cursp()));
+      }
       genop_peep(s, MKOP_AB(OP_RETURN, cursp(), OP_R_NORMAL), NOVAL);
     }
     if (val) push();