A USING clause when altering the type of a generated column does not
make sense. It would write the output of the USING clause into the
converted column, which would violate the generation expression.
This patch adds a check to error out if this is specified.
There was a test for this, but that test errored out for a different
reason, so it was not effective.
discovered by Jian He at [0]
[0]:
https://www.postgresql.org/message-id/CACJufxEGPYtFe79hbsMeOBOivfNnPRsw7Gjvk67m1x2MQggyiQ@mail.gmail.com
