Window functions: Support for aggregate_func(DISTINCT ...) OVER (...)

Should we support DISTINCT for aggregate functions that are computed over a window?

PostgreSQL doesn't support this:

test=#   select a, count(distinct b) over (order by a) from t1 ;

ERROR:  DISTINCT is not implemented for window functions

LINE 1: select a, count(distinct b) over (order by a) from t1 ;

                  ^

http://www.postgresql.org/docs/9.5/static/sql-expressions.html says "Aggregate window functions, unlike normal aggregate functions, do not allow DISTINCT or ORDER BY to be used within the function argument list"

SQL Server doesn't support it:
https://connect.microsoft.com/SQLServer/feedback/details/254393/over-clause-enhancement-request-distinct-clause-for-aggregate-functions
(verified with SQL Server 2008 and 2014)
but one can find many requests for them to support this.

Sybase documentation says:

No aggregation functions in the rank query are allowed to specify DISTINCT

http://infocenter.sybase.com/help/index.jsp?topic=/com.sybase.infocenter.dc38151.1510/html/iqrefbb/BCGEEBHA.htm
Does this mean they don't support it?

Oracle 12.1 gives this error

ORA-30487: ORDER BY not allowed here

Apache Drill DOESNT support it:
https://issues.apache.org/jira/browse/DRILL-3182