BUG #17579: 15beta2: strange error when trying to use MERGE statement as a CTE

The following bug has been logged on the website:

Bug reference: 17579
Logged by: Alexey Borzov
Email address: borz_off(at)cs(dot)msu(dot)su
PostgreSQL version: Unsupported/Unknown
Operating system: Windows
Description:

create table foo (id int);

with cte_failure as (
merge into foo as target
using foo as source
on target.id = source.id
when matched then do nothing
)
select 'fail!';

When executing the above code I get the following error:

> ERROR: DO INSTEAD NOTIFY rules are not supported for data-modifying
statements in WITH

I suspect that MERGE was never intended to work as a CTE, but right now the
grammar allows any PreparableStmt in a common_table_expr and the above error
is triggered a lot later due to implementation details.