slices: preserve nil/non-nil in Clone

Fixes golang/go#49660

Change-Id: I2bc371fed2193293509573d8994c94775edcf25e
Reviewed-on: https://go-review.googlesource.com/c/exp/+/365174
Trust: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Robert Griesemer <gri@golang.org>

slices/slices.go

@@ -156,7 +156,11 @@ func Delete[S ~[]E, E any](s S, i, j int) S {
 // Clone returns a copy of the slice.
 // The elements are copied using assignment, so this is a shallow clone.
 func Clone[S ~[]E, E any](s S) S {
-	return append(S(nil), s...)
+	// Preserve nil in case it matters.
+	if s == nil {
+		return nil
+	}
+	return append(S([]E{}), s...)
 }
 
 // Compact replaces consecutive runs of equal elements with a single copy.

slices/slices_test.go

@@ -457,6 +457,12 @@ func TestClone(t *testing.T) {
 	if !Equal(s2, want) {
 		t.Errorf("Clone(%v) changed unexpectedly to %v", want, s2)
 	}
+	if got := Clone([]int(nil)); got != nil {
+		t.Errorf("Clone(nil) = %#v, want nil", got)
+	}
+	if got := Clone(s1[:0]); got == nil || len(got) != 0 {
+		t.Errorf("Clone(%v) = %#v, want %#v", s1[:0], got, s1[:0])
+	}
 }
 
 var compactTests = []struct {