Why "alias X conflicts with an alias in the same statement" on aliased CTE use but not aliased table use? - firebird

Question

I wonder why the below statement causes this error with at least the Win32 Firebird WI-V2.5.7.27050 and WI-V2.5.2.26540 versions.
Dynamic SQL Error SQL error code = -204 alias TRIPLEDIGITS conflicts with an alias in the same statement
Unlinke many errors in SQL statements, the above error does not indicate the line that causes it, but with some fiddling I found the line marked with a comment causes it.
Query having one CTE unaliased and an the same CTE aliased:
with
recursive
tripledigits(n) as (
select 0
from rdb$database
union all
select tripledigits.n + 1
from tripledigits
where tripledigits.n < 999
),
sextupledigits(n) as (
select tripledigits.n
+ tripledigits000.n * 1000
from tripledigits
cross join tripledigits tripledigits000 -- causes "Dynamic SQL Error SQL error code = -204 alias TRIPLEDIGITS conflicts with an alias in the same statement"
order by tripledigits.n
+ tripledigits000.n * 1000
)
select sextupledigits.n
from sextupledigits
order by sextupledigits.n
It also fails when moving the failing query out of the sextupledigits CTE into the main query:
with
recursive
tripledigits(n) as (
select 0
from rdb$database
union all
select tripledigits.n + 1
from tripledigits
where tripledigits.n < 999
)
select tripledigits.n
+ tripledigits000.n * 1000
from tripledigits
cross join tripledigits tripledigits000 -- causes "Dynamic SQL Error SQL error code = -204 alias TRIPLEDIGITS conflicts with an alias in the same statement"
order by tripledigits.n
+ tripledigits000.n * 1000
Whereas using the below using an unaliased table and the same table aliased:
select rdb$database.rdb$relation_id + rd.rdb$relation_id * 1000
from rdb$database
cross join rdb$database rd
Why is my CTE based query throwing the error?
I know at least two workarounds (see below), but for documentation purposes and posterity I'd like an answer to the question.
Workaround 1: alias both CTE uses
with
recursive
tripledigits(n) as (
select 0
from rdb$database
union all
select tripledigits.n + 1
from tripledigits
where tripledigits.n < 999
),
sextupledigits(n) as (
select tripledigitsBare.n
+ tripledigits000.n * 1000
from tripledigits tripledigitsBare
cross join tripledigits tripledigits000
order by tripledigitsBare.n
+ tripledigits000.n * 1000
)
select sextupledigits.n
from sextupledigits
order by sextupledigits.n
Workaround 2: use an intermediate CTE
with
recursive
tripledigits(n) as (
select 0
from rdb$database
union all
select tripledigits.n + 1
from tripledigits
where tripledigits.n < 999
),
tripledigits000(n) as (
select tripledigits.n * 1000
from tripledigits
),
sextupledigits(n) as (
select tripledigits.n
+ tripledigits000.n
from tripledigits
cross join tripledigits000
order by tripledigits.n
+ tripledigits000.n
)
select sextupledigits.n
from sextupledigits
order by sextupledigits.n
I've reported it in the Firebird tracker as CORE-5519.