r.Shuffle(n, func(i, j int) { t.Fatalf("swap called, n=%d i=%d j=%d", n, i, j) })
	}
}

// encodePerm converts from a permuted slice of length n, such as Perm generates, to an int in [0, n!).
// See https://en.wikipedia.org/wiki/Lehmer_code.
// encodePerm modifies the input slice.
func encodePerm(s []int) int {
	// Convert to Lehmer code.
	for i, x := range s {
		r := s[i+1:]
		for j, y := range r {

		r.Shuffle(n, func(i, j int) { t.Fatalf("swap called, n=%d i=%d j=%d", n, i, j) })
	}
}

// encodePerm converts from a permuted slice of length n, such as Perm generates, to an int in [0, n!).
// See https://en.wikipedia.org/wiki/Lehmer_code.
// encodePerm modifies the input slice.
func encodePerm(s []int) int {
	// Convert to Lehmer code.
	for i, x := range s {
		r := s[i+1:]
		for j, y := range r {

// See Issue 16204.
func TestCatchInvalidNilValue(t *testing.T) {
	encodeErr, panicErr := encodeAndRecover(nil)
	if panicErr != nil {
		t.Fatalf("panicErr=%v, should not panic encoding untyped nil", panicErr)
	}
	if encodeErr == nil {
		t.Errorf("got err=nil, want non-nil error when encoding untyped nil value")
	} else if !strings.Contains(encodeErr.Error(), "nil value") {