Name resolution doesn't currently work when window functions are used with group by. Here is an example:

create table t1 (

  username  varchar(32),

  amount int

);

insert into t1 values

('user1',1),

('user1',5),

('user1',3),

('user2',10),

('user2',20),

('user2',30);

select 

  username,

  sum(amount), 

  rank() over (order by sum(amount) desc)

from t1

group by username;

The query is valid, but I get this error:

ERROR 1111 (HY000): Invalid use of group function

The above query does not comply with SQL standard. But it can be rewritten:

select 

  username,

  sum(amount) as s 

  rank() over (order by s desc)

from t1

group by username;

Yet it still returned the same error message.
I fixed this problem: now aliases for expressions with set functions can be used in window specifications.

The query may be non-standard, but PostgreSQL, SQL Server, and Oracle all support it.

I agree that we could live with the aliases, for now.