Correct the yeild var input bug.

src/graph/optimizer/rule/CollapseProjectRule.cpp

@@ -48,6 +48,10 @@ StatusOr<OptRule::TransformResult> CollapseProjectRule::transform(
   // 1. collect all property reference
   std::vector<std::string> allPropRefNames;
   for (auto col : colsAbove) {
+    if (graph::ExpressionUtils::findAny(
+            col->expr(), {Expression::Kind::kVar, Expression::Kind::kVersionedVar}) != nullptr) {
+      return TransformResult::noTransform();
+    }
     std::vector<const Expression*> propRefs = graph::ExpressionUtils::collectAll(
         col->expr(), {Expression::Kind::kVarProperty, Expression::Kind::kInputProperty});
     for (auto* expr : propRefs) {

src/graph/validator/YieldValidator.cpp

@@ -37,22 +37,20 @@ Status YieldValidator::validateImpl() {
 
   if (!exprProps_.srcTagProps().empty() || !exprProps_.dstTagProps().empty() ||
       !exprProps_.edgeProps().empty()) {
-    return Status::SemanticError("Only support input and variable in yield sentence.");
+    return Status::SemanticError("Not support vertex/edge property expression in yield sentence.");
   }
 
-  if (!exprProps_.inputProps().empty() && !exprProps_.varProps().empty()) {
+  if (!exprProps_.inputProps().empty() && !userDefinedVarNameList_.empty()) {
     return Status::SemanticError("Not support both input and variable.");
   }
 
-  if (!exprProps_.varProps().empty() && exprProps_.varProps().size() > 1) {
-    return Status::SemanticError("Only one variable allowed to use.");
+  if (userDefinedVarNameList_.size() > 1) {
+    // Now disable yield multiple user defined input for that means implicit dataset join
+    // As the inner variable take care don't make it a dataset
+    return Status::SemanticError("Multiple variables not supported yet.");
   }
 
-  if (!exprProps_.varProps().empty() && !userDefinedVarNameList_.empty()) {
-    // TODO: Support Multiple userDefinedVars
-    if (userDefinedVarNameList_.size() != 1) {
-      return Status::SemanticError("Multiple user defined vars not supported yet.");
-    }
+  if (!userDefinedVarNameList_.empty()) {
     userDefinedVarName_ = *userDefinedVarNameList_.begin();
   }
 

src/graph/validator/test/YieldValidatorTest.cpp

@@ -436,7 +436,8 @@ TEST_F(YieldValidatorTest, Error) {
     // Not support reference two different variable
     auto query = var + "YIELD $var.name WHERE $var1.start > 2005";
     auto result = checkResult(query);
-    EXPECT_EQ(std::string(result.message()), "SemanticError: Only one variable allowed to use.");
+    EXPECT_EQ(std::string(result.message()),
+              "SemanticError: Multiple variables not supported yet.");
   }
   {
     // Reference a non-existed prop is meaningless.
@@ -453,13 +454,13 @@ TEST_F(YieldValidatorTest, Error) {
     auto query = var + "YIELD $$.person.name";
     auto result = checkResult(query);
     EXPECT_EQ(std::string(result.message()),
-              "SemanticError: Only support input and variable in yield sentence.");
+              "SemanticError: Not support vertex/edge property expression in yield sentence.");
   }
   {
     auto query = var + "YIELD $^.person.name";
     auto result = checkResult(query);
     EXPECT_EQ(std::string(result.message()),
-              "SemanticError: Only support input and variable in yield sentence.");
+              "SemanticError: Not support vertex/edge property expression in yield sentence.");
   }
   {
     auto query = var + "YIELD like.start";

src/graph/visitor/DeducePropsVisitor.cpp

@@ -137,7 +137,10 @@ void DeducePropsVisitor::visit(InputPropertyExpression *expr) {
 
 void DeducePropsVisitor::visit(VariablePropertyExpression *expr) {
   exprProps_->insertVarProp(expr->sym(), expr->prop());
-  userDefinedVarNameList_->emplace(expr->sym());
+  if (expr->sym()[0] != '_') {
+    // exclude inner generated variable
+    userDefinedVarNameList_->emplace(expr->sym());
+  }
 }
 
 void DeducePropsVisitor::visit(DestPropertyExpression *expr) {
@@ -168,7 +171,12 @@ void DeducePropsVisitor::visit(EdgeDstIdExpression *expr) { visitEdgePropExpr(ex
 
 void DeducePropsVisitor::visit(UUIDExpression *expr) { reportError(expr); }
 
-void DeducePropsVisitor::visit(VariableExpression *expr) { UNUSED(expr); }
+void DeducePropsVisitor::visit(VariableExpression *expr) {
+  if (expr->var()[0] != '_') {
+    // exclude inner generated variable
+    userDefinedVarNameList_->emplace(expr->var());
+  }
+}
 
 void DeducePropsVisitor::visit(VersionedVariableExpression *expr) { reportError(expr); }
 

tests/tck/features/bugfix/YieldVar.feature

@@ -0,0 +1,19 @@
+# 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: Test yield var
+
+  Background:
+    Given a graph with space named "nba"
+
+  # #2646
+  Scenario: yield constant after pipe
+    When executing query:
+      """
+      $var = GO FROM "Tim Duncan" OVER like YIELD like._dst; YIELD $var[0][0] as var00;
+      """
+    Then the result should be, in any order:
+      | var00           |
+      | "Manu Ginobili" |
+      | "Manu Ginobili" |

tests/tck/features/yield/yield.IntVid.feature

@@ -288,7 +288,7 @@ Feature: Yield Sentence
       """
       $var = GO FROM hash("Boris Diaw") OVER serve YIELD $^.player.name AS name, serve.start_year AS start, $$.team.name AS team;YIELD $var.team WHERE $var1.start > 2005
       """
-    Then a SemanticError should be raised at runtime: Only one variable allowed to use.
+    Then a SemanticError should be raised at runtime: Multiple variables not supported yet.
     When executing query:
       """
       $var = GO FROM hash("Boris Diaw") OVER serve YIELD $^.player.name AS name, serve.start_year AS start, $$.team.name AS team;YIELD $var.abc

tests/tck/features/yield/yield.feature

@@ -298,7 +298,7 @@ Feature: Yield Sentence
       """
       $var = GO FROM "Boris Diaw" OVER serve YIELD $^.player.name AS name, serve.start_year AS start, $$.team.name AS team;YIELD $var.team WHERE $var1.start > 2005
       """
-    Then a SemanticError should be raised at runtime: Only one variable allowed to use.
+    Then a SemanticError should be raised at runtime: Multiple variables not supported yet.
     When executing query:
       """
       $var = GO FROM "Boris Diaw" OVER serve YIELD $^.player.name AS name, serve.start_year AS start, $$.team.name AS team;YIELD $var.abc