disable yield var

src/parser/parser.yy

@@ -1436,9 +1436,16 @@ yield_column
         delete $3;
     }
     | expression {
+        if (!graph::ExpressionUtils::collectAll($1, {Expression::Kind::kVar}).empty()) {
+            throw nebula::GraphParser::syntax_error(@1, "Direct output of variable is prohibited");
+        }
         $$ = new YieldColumn($1);
     }
     | expression KW_AS name_label {
+        if (!graph::ExpressionUtils::collectAll($1, {Expression::Kind::kVar}).empty()) {
+            delete $3;
+            throw nebula::GraphParser::syntax_error(@1, "Direct output of variable is prohibited");
+        }
         $$ = new YieldColumn($1, *$3);
         delete $3;
     }

tests/tck/features/bugfix/VariableExpression.feature

@@ -0,0 +1,26 @@
+# Copyright (c) 2021 vesoft inc. All rights reserved.
+#
+# This source code is licensed under Apache 2.0 License,
+# attached with Common Clause Condition 1.0, found in the LICENSES directory.
+Feature: Variable usage
+
+  Background:
+    Given a graph with space named "nba"
+
+  Scenario: disable yield $var
+    When executing query:
+      """
+      $var = yield 1;$var2 = yield 3;yield $var1 + $var2
+      """
+    Then a SyntaxError should be raised at runtime: Direct output of variable is prohibited near `$var1 + $var2'
+    When executing query:
+      """
+      $var=go from "Tim Duncan" over like yield like._dst as dst;yield $var
+      """
+    Then a SyntaxError should be raised at runtime: Direct output of variable is prohibited near `$var'
+    Then drop the used space
+    When executing query:
+      """
+      $var=go from "Tim Duncan" over like yield like._dst as dst;yield $var[0][0]
+      """
+    Then a SyntaxError should be raised at runtime: Direct output of variable is prohibited near `$var[0][0]'