def cast(db, type_, col, errors_table=None):

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

    If a column, converts any errors to warnings.

    @param col sql_gen.Col|sql_gen.Literal

    @param errors_table None|sql_gen.Table|str

        If set and col is a column with srcs, saves any errors in this table,

        using column's srcs attr as the source columns.

    '''

    if isinstance(col, sql_gen.Literal): # literal value, so just cast

        return sql_gen.CustomCode(col.to_str(db)+'::'+type_)

    assert isinstance(col, sql_gen.Col)

    assert not isinstance(col, sql_gen.NamedCol)

    save_errors = errors_table != None and col.srcs != ()

    if save_errors:

        errors_table = sql_gen.as_Table(errors_table)

        srcs = map(sql_gen.to_name_only_col, col.srcs)

        function_name = str(sql_gen.FunctionCall(type_, srcs))

    else: function_name = type_

    function = sql_gen.TempFunction(function_name, db.autocommit)

    query = '''\

CREATE FUNCTION '''+function.to_str(db)+'''(value text)

RETURNS '''+type_+'''

LANGUAGE plpgsql

'''

    if not save_errors: query += 'IMMUTABLE\n'

    query += '''\

STRICT

AS $$

BEGIN

    /* The explicit cast to the return type is needed to make the cast happen

    inside the try block. (Implicit casts to the return type happen at the end

    of the function, outside any block.) */

    RETURN value::'''+type_+''';

EXCEPTION

    WHEN data_exception THEN

'''

    if save_errors:

        col_names = map(sql_gen.Literal, srcs)

        query += '''\

        -- Save error in errors table.

        BEGIN

            INSERT INTO '''+errors_table.to_str(db)+'''

            ("column", value, error)

            (VALUES '''+(', '.join(('('+c.to_str(db)+')' for c in col_names))

                )+''') AS c

            CROSS JOIN

            (VALUES (value, SQLERRM)) AS v

            ;

        EXCEPTION

            WHEN unique_violation THEN NULL; -- ignore duplicate key

        END;

'''

    query += '''\

        RAISE WARNING '%', SQLERRM;

        RETURN NULL;

END;

$$;

'''

    try:

        run_query(db, query, recover=True, cacheable=True,

            log_ignore_excs=(DuplicateFunctionException,))

    except DuplicateFunctionException: pass # function already existed

    return sql_gen.FunctionCall(function, col)