as :class:`.TypeDecorator` objects.

dialect on a type where MySQL does not support CAST; MySQL only
supports CAST on a subset of datatypes.   SQLAlchemy has for a long
time just omitted the CAST for unsupported types in the case of
MySQL.  While we don't want to change this now, we emit a warning
to show that it's taken place.   A warning is also emitted when
a CAST is used with an older MySQL version (< 4) that doesn't support
CAST at all, it's skipped in this case as well.
fixes #3237

commit of pr152

on jenkins

all cases, so that MySQL 5.6.6 with the
``explicit_defaults_for_timestamp`` flag enabled will
will allow TIMESTAMP to continue to work as expected when
``nullable=False``.  Existing applications are unaffected as
SQLAlchemy has always emitted NULL for a TIMESTAMP column that
is ``nullable=True``.  fixes #3155

the ARRAY type when using psycopg2.  The psycopg2 dialect now
employs use of the psycopg2.extras.register_uuid() hook
so that UUID values are always passed to/from the DBAPI as
UUID() objects.   The :paramref:`.UUID.as_uuid` flag is still
honored, except with psycopg2 we need to convert returned
UUID objects back into strings when this is disabled.
fixes #2940