Failed Assertion about PolymorphicType

Hi,

Running sqlsmith on master i got an assertion failure on parse_coerce.c:2049

This is a minimal query to reproduce in an empty database, i also
attached the stack trace

 """
select
  pg_catalog.array_in(
    cast(pg_catalog.regoperatorout(
      cast(cast(null as regoperator) as regoperator)) as cstring),
    cast((select pronamespace from pg_catalog.pg_proc limit 1 offset 1)
       as oid),
    cast(subq_1.pid as int4)) as c0
from pg_catalog.pg_stat_progress_analyze as subq_1
 """

-- 
Jaime Casanova                      www.2ndQuadrant.com
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