fix panic if copying in exactly the buffer size

deque.go

@@ -320,7 +320,7 @@ func (q *Deque[T]) Copy(src Deque[T]) int {
 	q.Grow(src.Len())
 	n := src.CopyOutSlice(q.buf)
 	q.count = n
-	q.tail = n
+	q.tail = n & (len(q.buf) - 1) // bitwise modulus
 	q.head = 0
 	return n
 }
@@ -380,7 +380,7 @@ func (q *Deque[T]) CopyInSlice(in []T) {
 	}
 	n := copy(q.buf, in)
 	q.count = n
-	q.tail = n
+	q.tail = n & (len(q.buf) - 1) // bitwise modulus
 	q.head = 0
 }
 

deque_test.go

@@ -1275,6 +1275,34 @@ func TestCopy(t *testing.T) {
 	}
 }
 
+func TestClearAndPushBack(t *testing.T) {
+	q := &Deque[int]{}
+	in := make([]int, minCapacity)
+	q.Clear()
+	for i := range in {
+		q.PushBack(in[i])
+	}
+	q.PopFront()
+	q.PushBack(17)
+}
+
+func TestCopyPanic(t *testing.T) {
+	var q1, q2 Deque[int]
+	in := make([]int, minCapacity)
+	q1.CopyInSlice(in)
+	q2.Copy(q1)
+	q2.PopFront()
+	q2.PushBack(17)
+}
+
+func TestCopyInSlicePanic(t *testing.T) {
+	var q Deque[int]
+	in := make([]int, minCapacity)
+	q.CopyInSlice(in)
+	q.PopFront()
+	q.PushBack(17)
+}
+
 func assertPanics(t *testing.T, name string, f func()) {
 	defer func() {
 		if r := recover(); r == nil {