Most of the infrastructure for procedure arguments was already
okay with polymorphic output arguments, but it turns out that
CallStmtResultDesc() was a few bricks shy of a load here. It thought
all it needed to do was call build_function_result_tupdesc_t, but
that function specifically disclaims responsibility for resolving
polymorphic arguments. Failing to handle that doesn't seem to be
a problem for CALL in plpgsql, but CALL from plain SQL would get
errors like "cannot display a value of type anyelement", or even
crash outright.
In v14 and later we can simply examine the exposed types of the
CallStmt.outargs nodes to get the right type OIDs. But it's a lot
more complicated to fix in v12/v13, because those versions don't
have CallStmt.outargs, nor do they do expand_function_arguments
until ExecuteCallStmt runs. We have to duplicatively run
expand_function_arguments, and then re-determine which elements
of the args list are output arguments.
Per bug #18463 from Drew Kimball. Back-patch to all supported
versions, since it's busted in all of them.
Discussion: https://postgr.es/m/18463-f8cd77e12564d8a2@postgresql.org
f535f350c1