Query: Join after GroupByAggregate throws when join key uses grouping key or aggregate function

something like
```C#
var query = context.Orders.GroupBy(o => o.CustomerID).Select(g => new { g.Key, Count = g.Count() })
.Join(context.Customers, o => o.Key, c => c.CustomerID, (o, c) => new {C = c, o.Count});
```