What exactly does "No Join Predicate" mean in SQL Server?
Your question is similar to this one. Sometimes SQL Server can remove a join predicate from the original query.

In the case where you see a join predicate warning SQL Server detects at compile time that the table of constants only has one distinct value and that value is 1 so rewrites the query as

SELECT *
FROM   (SELECT *
        FROM   #temp1 t1
        WHERE  t1.i = 1) t1
       CROSS JOIN (SELECT 1 i
                   UNION ALL
                   SELECT 1) t2 


There is a predicate on the table scan of #temp as follows [tempdb].[dbo].[#temp1].[i] =(1)

The join predicate of on t1.i = t2.i can't be removed in this way at compile time when using two tables or if the table of constants contains more than one distinct value.



More info about this can be found in Paul White's Query Optimizer Deep Dive series.