'''Casts an (unrenamed) column or value.

    If errors_table set and col has srcs, saves errors in errors_table (using

    col's srcs attr as the source columns) and converts errors to warnings.

    @param errors_table None|sql_gen.Table|str

    '''

    save_errors = (errors_table != None and isinstance(col, sql_gen.Col)

        and col.srcs != ())

    if not save_errors: # can't save errors