no decoder for composite type element in position _ of type OID _

• asyncpg version: 0.17.0
• PostgreSQL version: 10.5
• Do you use a PostgreSQL SaaS? If so, which? Can you reproduce
  the issue with a local PostgreSQL install?:
• Python version: 3.7.0
• Platform: Linux
• Do you use pgbouncer?: No
• Did you install asyncpg with pip?: pipenv
• If you built asyncpg locally, which version of Cython did you use?:
• Can the issue be reproduced under both asyncio and
  uvloop?: asyncio definitely, untested with uvloop

Whenever I am trying to call a plpgsql stored procedure which returns a custom defined enum, I receive the following error:
RuntimeError: no decoder for composite type element in position 2 of type OID 88492

Type with OID 88492 in my database (from pg_catalog.pg_type) is listed as ssg.logintype which is defined as below:

	CREATE TYPE SSG.LOGINTYPE AS ENUM (
		'phone',
		'email'
	);

Partial traceback (asyncpg relevant parts):

  File "/home/mayzie/.local/share/virtualenvs/api-xyfAGMEp/lib/python3.7/site-packages/asyncpg/prepared_stmt.py", line 179, in fetchval
    data = await self.__bind_execute(args, 1, timeout)
  File "/home/mayzie/.local/share/virtualenvs/api-xyfAGMEp/lib/python3.7/site-packages/asyncpg/prepared_stmt.py", line 203, in __bind_execute
    self._state, args, '', limit, True, timeout)
  File "asyncpg/protocol/protocol.pyx", line 212, in bind_execute
  File "asyncpg/protocol/coreproto.pyx", line 71, in asyncpg.protocol.protocol.CoreProtocol._read_server_messages
  File "asyncpg/protocol/coreproto.pyx", line 205, in asyncpg.protocol.protocol.CoreProtocol._process__bind_execute
  File "asyncpg/protocol/coreproto.pyx", line 495, in asyncpg.protocol.protocol.CoreProtocol._parse_data_msgs
  File "asyncpg/protocol/protocol.pyx", line 770, in asyncpg.protocol.protocol.BaseProtocol._decode_row
  File "asyncpg/protocol/prepared_stmt.pyx", line 286, in asyncpg.protocol.protocol.PreparedStatementState._decode_row
  File "asyncpg/protocol/codecs/base.pyx", line 257, in asyncpg.protocol.protocol.Codec.decode
  File "asyncpg/protocol/codecs/base.pyx", line 166, in asyncpg.protocol.protocol.Codec.decode_scalar
  File "asyncpg/protocol/codecs/record.pyx", line 39, in asyncpg.protocol.protocol.anonymous_record_decode
RuntimeError: no decoder for composite type element in position 2 of type OID 88492

protocol/codecs/record.pyx に示されているデコードパスと、traceback に記載されている周辺の prepared-statement および codec ファイルから始めます。PostgreSQL 10.5、カスタム enum、複合値を使って stored procedure の結果を再現し、その後、enum 要素に decoder がない理由を追跡します。報告された RuntimeError なしで結果をデコードでき、その動作が回帰テストでカバーされれば完了です。