I added an error check for switch on a func/map/slice to
ensure that the only valid case is nil.  The error print says

    yyerror("invalid case %N in switch (can only compare %lN to nil)", ll->n, n->ntest);

and then the error says

    bytes_test.go:698: invalid case bytes.Trim in switch (can only compare tc.f (type func(? []byte, ? string) []byte) to nil)

The question marks should not be there.