slices.Delete: fix behavior on invalid slice indices arguments

Now doing an explicit bounds check, to panic on invalid indices

Fixes part (3) of golang/go#54650

Change-Id: I62a70c0f866e6c336d0c7feae5af33272fc568b6
Reviewed-on: https://go-review.googlesource.com/c/exp/+/425894
Auto-Submit: Keith Randall <[email protected]>
TryBot-Result: Gopher Robot <[email protected]>
Run-TryBot: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>
Reviewed-by: Keith Randall <[email protected]>

Author: Deleplace

slices/slices.go

@@ -154,6 +154,8 @@ func Insert[S ~[]E, E any](s S, i int, v ...E) S {
 // Delete is O(len(s)-(j-i)), so if many items must be deleted, it is better to
 // make a single call deleting them all together than to delete one at a time.
 func Delete[S ~[]E, E any](s S, i, j int) S {
+	_ = s[i:j] // bounds check
+
 	return append(s[:i], s[j:]...)
 }
 

slices/slices_test.go

@@ -447,6 +447,34 @@ func TestDelete(t *testing.T) {
 	}
 }
 
+func panics(f func()) (b bool) {
+	defer func() {
+		if x := recover(); x != nil {
+			b = true
+		}
+	}()
+	f()
+	return false
+}
+
+func TestDeletePanics(t *testing.T) {
+	for _, test := range []struct {
+		name string
+		s    []int
+		i, j int
+	}{
+		{"with negative first index", []int{42}, -2, 1},
+		{"with negative second index", []int{42}, 1, -1},
+		{"with out-of-bounds first index", []int{42}, 2, 3},
+		{"with out-of-bounds second index", []int{42}, 0, 2},
+		{"with invalid i>j", []int{42}, 1, 0},
+	} {
+		if !panics(func() { Delete(test.s, test.i, test.j) }) {
+			t.Errorf("Delete %s: got no panic, want panic", test.name)
+		}
+	}
+}
+
 func TestClone(t *testing.T) {
 	s1 := []int{1, 2, 3}
 	s2 := Clone(s1)