Migrated issue, originally created by frol (@frol)

Having a custom column type, e.g.:

class PasswordType(types.TypeDecorator, ScalarCoercible):
    def load_dialect_impl(self, dialect):
        if dialect.name == 'sqlite':
            # Use a NUMERIC type for sqlite
            impl = sqlite.NUMERIC(self.length)
        else:
            # Use a VARBINARY for all other dialects.
            impl = types.VARBINARY(self.length)
        return dialect.type_descriptor(impl)

Alembic crashes when tries to compare the existing (the first migration is fine) column with the custom type:

  File ".../lib/python3.5/site-packages/alembic/autogenerate/compare.py", line 77, in _autogen_for_tables
    inspector, metadata, upgrade_ops, autogen_context)
  File ".../lib/python3.5/site-packages/alembic/autogenerate/compare.py", line 177, in _compare_tables
    modify_table_ops, autogen_context, inspector):
  File ".../lib/python3.5/contextlib.py", line 59, in __enter__
    return next(self.gen)
  File ".../lib/python3.5/site-packages/alembic/autogenerate/compare.py", line 256, in _compare_columns
    schema, tname, colname, conn_col, metadata_col
  File ".../lib/python3.5/site-packages/alembic/util/langhelpers.py", line 314, in go
    fn(*arg, **kw)
  File ".../lib/python3.5/site-packages/alembic/autogenerate/compare.py", line 624, in _compare_type
    conn_col, metadata_col)
  File ".../lib/python3.5/site-packages/alembic/runtime/migration.py", line 391, in _compare_type
    metadata_column)
  File ".../lib/python3.5/site-packages/alembic/ddl/impl.py", line 261, in compare_type
    return comparator and comparator(metadata_type, conn_type)
  File ".../lib/python3.5/site-packages/alembic/ddl/impl.py", line 335, in _numeric_compare
    t1.precision is not None and
  File ".../lib/python3.5/site-packages/sqlalchemy/sql/type_api.py", line 913, in __getattr__
    return getattr(self.impl, key)
AttributeError: 'VARBINARY' object has no attribute 'precision'

I have attempted to fix this in PR: https://bitbucket.org/zzzeek/alembic/pull-requests/64/