slices: make Insert panic if index is out of range and there are no values

Fixes #63913

Change-Id: I514190b104a2c4bd5a6b0d96659b52904185e91f
GitHub-Last-Rev: 90e7195193
GitHub-Pull-Request: golang/go#63965
Reviewed-on: https://go-review.googlesource.com/c/go/+/540155
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Heschi Kreinick <heschi@google.com>
Run-TryBot: Jes Cok <xigua67damn@gmail.com>
Reviewed-by: Keith Randall <khr@google.com>

src/slices/slices.go

@@ -130,11 +130,14 @@ func ContainsFunc[S ~[]E, E any](s S, f func(E) bool) bool {
 // Insert panics if i is out of range.
 // This function is O(len(s) + len(v)).
 func Insert[S ~[]E, E any](s S, i int, v ...E) S {
+	n := len(s)
 	m := len(v)
 	if m == 0 {
+		// Panic if i is not in the range [0:n] inclusive.
+		// See issue 63913.
+		_ = s[:n:n][i:]
 		return s
 	}
-	n := len(s)
 	if i == n {
 		return append(s, v...)
 	}

src/slices/slices_test.go

@@ -536,6 +536,26 @@ func TestInsertOverlap(t *testing.T) {
 	}
 }
 
+func TestInsertPanics(t *testing.T) {
+	a := [3]int{}
+	for _, test := range []struct {
+		name string
+		s    []int
+		i    int
+		v    []int
+	}{
+		// There are no values.
+		{"with negative index", a[:1:1], -1, nil},
+		{"with out-of-bounds index and > cap", a[:1:1], 2, nil},
+		{"with out-of-bounds index and = cap", a[:1:2], 2, nil},
+		{"with out-of-bounds index and < cap", a[:1:3], 2, nil},
+	} {
+		if !panics(func() { Insert(test.s, test.i, test.v...) }) {
+			t.Errorf("Insert %s: got no panic, want panic", test.name)
+		}
+	}
+}
+
 var deleteTests = []struct {
 	s    []int
 	i, j int