slices: update doc for Delete and Replace

Fixes #64013

Change-Id: Ibaeaad6120bff041bf6ab80fd4cd613f7d4ac5a7
GitHub-Last-Rev: 647ed646ec7c2e4ce93c5d3847d0b9e3627d7497
GitHub-Pull-Request: golang/go#64024
Reviewed-on: https://go-review.googlesource.com/c/go/+/540955
Reviewed-by: Keith Randall <khr@golang.org>
Run-TryBot: Jes Cok <xigua67damn@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@google.com>
Auto-Submit: Keith Randall <khr@golang.org>
Reviewed-by: Eli Bendersky <eliben@google.com>

2 files changed, 14 insertions, 3 deletions

diff --git a/src/slices/slices.go b/src/slices/slices.go
index fe50a91d48..4c398557ff 100644
--- a/src/slices/slices.go
+++ b/src/slices/slices.go
@@ -212,7 +212,7 @@ func Insert[S ~[]E, E any](s S, i int, v ...E) S {
 }
 
 // Delete removes the elements s[i:j] from s, returning the modified slice.
-// Delete panics if s[i:j] is not a valid slice of s.
+// Delete panics if j > len(s) or s[i:j] is not a valid slice of s.
 // Delete is O(len(s)-j), 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.
 // Delete might not modify the elements s[len(s)-(j-i):len(s)]. If those
@@ -246,9 +246,10 @@ func DeleteFunc[S ~[]E, E any](s S, del func(E) bool) S {
 }
 
 // Replace replaces the elements s[i:j] by the given v, and returns the
-// modified slice. Replace panics if s[i:j] is not a valid slice of s.
+// modified slice.
+// Replace panics if j > len(s) or s[i:j] is not a valid slice of s.
 func Replace[S ~[]E, E any](s S, i, j int, v ...E) S {
-	_ = s[i:j] // verify that i:j is a valid subslice
+	_ = s[i:j] // bounds check
 
 	if i == j {
 		return Insert(s, i, v...)
diff --git a/src/slices/slices_test.go b/src/slices/slices_test.go
index 7d4fc34b2e..2fc583ff90 100644
--- a/src/slices/slices_test.go
+++ b/src/slices/slices_test.go
@@ -659,6 +659,10 @@ func panics(f func()) (b bool) {
 }
 
 func TestDeletePanics(t *testing.T) {
+	s := []int{0, 1, 2, 3, 4}
+	s = s[0:2]
+	_ = s[0:4] // this is a valid slice of s
+
 	for _, test := range []struct {
 		name string
 		s    []int
@@ -669,6 +673,7 @@ func TestDeletePanics(t *testing.T) {
 		{"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},
+		{"s[i:j] is valid and j > len(s)", s, 0, 4},
 	} {
 		if !panics(func() { Delete(test.s, test.i, test.j) }) {
 			t.Errorf("Delete %s: got no panic, want panic", test.name)
@@ -928,6 +933,10 @@ func TestReplace(t *testing.T) {
 }
 
 func TestReplacePanics(t *testing.T) {
+	s := []int{0, 1, 2, 3, 4}
+	s = s[0:2]
+	_ = s[0:4] // this is a valid slice of s
+
 	for _, test := range []struct {
 		name string
 		s, v []int
@@ -936,6 +945,7 @@ func TestReplacePanics(t *testing.T) {
 		{"indexes out of order", []int{1, 2}, []int{3}, 2, 1},
 		{"large index", []int{1, 2}, []int{3}, 1, 10},
 		{"negative index", []int{1, 2}, []int{3}, -1, 2},
+		{"s[i:j] is valid and j > len(s)", s, nil, 0, 4},
 	} {
 		ss, vv := Clone(test.s), Clone(test.v)
 		if !panics(func() { Replace(ss, test.i, test.j, vv...) }) {