Throw when reduce inside exists (#1392)

src/query/frontend/semantic/symbol_generator.cpp

@@ -480,12 +480,20 @@ bool SymbolGenerator::PreVisit(None &none) {
 }
 
 bool SymbolGenerator::PreVisit(Reduce &reduce) {
+  auto &scope = scopes_.back();
+  scope.in_reduce = true;
   reduce.initializer_->Accept(*this);
   reduce.list_->Accept(*this);
   VisitWithIdentifiers(reduce.expression_, {reduce.accumulator_, reduce.identifier_});
   return false;
 }
 
+bool SymbolGenerator::PostVisit(Reduce & /*reduce*/) {
+  auto &scope = scopes_.back();
+  scope.in_reduce = false;
+  return true;
+}
+
 bool SymbolGenerator::PreVisit(Extract &extract) {
   extract.list_->Accept(*this);
   VisitWithIdentifiers(extract.expression_, {extract.identifier_});
@@ -496,15 +504,19 @@ bool SymbolGenerator::PreVisit(Exists &exists) {
   auto &scope = scopes_.back();
 
   if (scope.in_set_property) {
-    throw utils::NotYetImplemented("Set property can not be used with exists, but only during matching!");
+    throw utils::NotYetImplemented("Exists cannot be used within SET clause.!");
   }
 
   if (scope.in_with) {
-    throw utils::NotYetImplemented("WITH can not be used with exists, but only during matching!");
+    throw utils::NotYetImplemented("Exists cannot be used within WITH!");
   }
 
   if (scope.in_return) {
-    throw utils::NotYetImplemented("RETURN can not be used with exists, but only during matching!");
+    throw utils::NotYetImplemented("Exists cannot be used within RETURN!");
+  }
+
+  if (scope.in_reduce) {
+    throw utils::NotYetImplemented("Exists cannot be used within REDUCE!");
   }
 
   if (scope.num_if_operators) {

src/query/frontend/semantic/symbol_generator.hpp

@@ -84,6 +84,7 @@ class SymbolGenerator : public HierarchicalTreeVisitor {
   bool PreVisit(Any &) override;
   bool PreVisit(None &) override;
   bool PreVisit(Reduce &) override;
+  bool PostVisit(Reduce &) override;
   bool PreVisit(Extract &) override;
   bool PreVisit(Exists & /*exists*/) override;
   bool PostVisit(Exists & /*exists*/) override;
@@ -123,6 +124,7 @@ class SymbolGenerator : public HierarchicalTreeVisitor {
     bool in_match{false};
     bool in_foreach{false};
     bool in_exists{false};
+    bool in_reduce{false};
     bool in_set_property{false};
     bool in_call_subquery{false};
     bool has_return{false};

tests/gql_behave/tests/memgraph_V1/features/functions.feature

@@ -1158,3 +1158,15 @@ Feature: Functions
         Then the result should be:
             | A_COUNT | B_COUNT |
             | 3       | 15      |
+
+    Scenario: Exists is forbidden within reduce:
+      Given an empty graph
+      And having executed:
+        """
+        CREATE ()
+        """
+      When executing query:
+        """
+        MATCH () WHERE reduce(a=exists(()),b in []|a) RETURN 1;
+        """
+      Then an error should be raised