Merge command is erroring with: PlaceHolderVar found where not expected

Hi hackers,

While playing with the Merge command, I get an unexpected error. I've
searched the mailing list, but couldn't find any reference to this, so
decided to send an e-mail.

CREATE TABLE source (id integer, z int);
CREATE TABLE target (id integer, z int);
MERGE INTO public.target sda USING ((SELECT source.id, source.z, 12 AS rnd
FROM public.source source) sdn FULL JOIN public.source sdn2 ON ((sdn.id
OPERATOR(pg_catalog.=) sdn2.id))) ON (sda.id OPERATOR(pg_catalog.=) sdn.id)
WHEN NOT MATCHED THEN INSERT (id, z) VALUES (sdn.id, sdn.rnd);

*ERROR: PlaceHolderVar found where not expected*

Here is the backtrace:
```

thread #1, queue = 'com.apple.main-thread', stop reason = breakpoint 2.1
* frame #0: 0x000000010d60f0df postgres`errstart(elevel=21,
domain=0x0000000000000000) at elog.c:453:6 [opt]
frame #1: 0x000000010d6d7e7e postgres`pull_var_clause_walker.cold.7 at
var.c:703:4 [opt]
frame #2: 0x000000010d415dae
postgres`pull_var_clause_walker(node=<unavailable>,
context=0x00007ff7b2dcb7d0) at var.c:703:4 [opt]
frame #3: 0x000000010d3782a5
postgres`expression_tree_walker_impl(node=0x00007ff18503a7c0,
walker=<unavailable>, context=<unavailable>) at nodeFuncs.c:2206:9 [opt]
frame #4: 0x000000010d415dfe
postgres`pull_var_clause_walker(node=<unavailable>, context=<unavailable>)
at var.c:705:9 [opt] [artificial]
frame #5: 0x000000010d415cfc postgres`pull_var_clause(node=<unavailable>,
flags=0) at var.c:622:2 [opt]
frame #6: 0x000000010d3fac81
postgres`preprocess_targetlist(root=0x00007ff185037a28) at
preptlist.c:165:11 [opt]
frame #7: 0x000000010d3e6941 postgres`grouping_planner(root=<unavailable>,
tuple_fraction=<unavailable>) at planner.c:1419:3 [opt]
frame #8: 0x000000010d3e5678 postgres`subquery_planner(glob=<unavailable>,
parse=<unavailable>, parent_root=<unavailable>, hasRecursion=<unavailable>,
tuple_fraction=0) at planner.c:1065:2 [opt]
frame #9: 0x000000010d3e4277
postgres`standard_planner(parse=0x00007ff185011668,
query_string=<unavailable>, cursorOptions=2048,
boundParams=0x0000000000000000) at planner.c:411:9 [opt]
```

Thanks,
Onder KALACI