Fixed CORE-2584 - Wrong results for CASE used together with GROUP BY

src/dsql/gen.cpp

@@ -2729,8 +2729,12 @@ static void gen_simple_case( CompiledStatement* statement, const dsql_nod* node)
 	{
 		stuff(statement, blr_value_if);
 		stuff(statement, blr_eql);
-		GEN_expr(statement, node->nod_arg[(wptr == when_list->nod_arg ?
-			e_simple_case_case_operand : e_simple_case_case_operand2)]);
+
+		if (wptr == when_list->nod_arg || !node->nod_arg[e_simple_case_case_operand2])
+			GEN_expr(statement, node->nod_arg[e_simple_case_case_operand]);
+		else
+			GEN_expr(statement, node->nod_arg[e_simple_case_case_operand2]);
+
 		GEN_expr(statement, *wptr);
 		GEN_expr(statement, *rptr);
 	}

src/dsql/pass1.cpp

@@ -8318,15 +8318,16 @@ static dsql_nod* pass1_simple_case( CompiledStatement* statement, dsql_nod* inpu
 	DEV_BLKCHK(input->nod_arg[0], dsql_type_nod);
 
 	dsql_nod* node = MAKE_node(nod_simple_case, 4);
-	node->nod_count = 3;	// we do not want to reprocess last parameter later
+	node->nod_count = 3;	// we do not want to reprocess e_simple_case_case_operand2 later
 
 	// build case_operand node
 	node->nod_arg[e_simple_case_case_operand] = PASS1_node(statement, input->nod_arg[0]);
-	node->nod_arg[e_simple_case_case_operand2] =
-		pass1_hidden_variable(statement, node->nod_arg[e_simple_case_case_operand]);
+	node->nod_arg[e_simple_case_case_operand2] = pass1_hidden_variable(
+		statement, node->nod_arg[e_simple_case_case_operand]);
 
-	if (!node->nod_arg[e_simple_case_case_operand2])
-		node->nod_arg[e_simple_case_case_operand2] = node->nod_arg[e_simple_case_case_operand];
+	// If a hidden variable was generated, it will be stored in e_simple_case_case_operand2 and
+	// we'll use it in GEN. Otherwise, e_simple_case_case_operand2 is NULL and we should use the
+	// original expression (e_simple_case_case_operand) in GEN.
 
 	dsql_nod* list = input->nod_arg[1];