Fix runtime error not allowing to slice unaddressable array type (#803)

* fix runtime error not allowing to slice unaddressable array type

* simplify logic and move test

* remove dead code

---------

Signed-off-by: Anton Medvedev <anton@medv.io>
Co-authored-by: Anton Medvedev <anton@medv.io>

Author: diegommm

expr_test.go

@@ -2846,6 +2846,23 @@ func TestMemoryBudget(t *testing.T) {
 	}
 }
 
+func TestIssue802(t *testing.T) {
+	prog, err := expr.Compile(`arr[1:2][0]`)
+	if err != nil {
+		t.Fatalf("error compiling program: %v", err)
+	}
+	val, err := expr.Run(prog, map[string]any{
+		"arr": [5]int{0, 1, 2, 3, 4},
+	})
+	if err != nil {
+		t.Fatalf("error running program: %v", err)
+	}
+	valInt, ok := val.(int)
+	if !ok || valInt != 1 {
+		t.Fatalf("invalid result, expected 1, got %v", val)
+	}
+}
+
 func TestIssue807(t *testing.T) {
 	type MyStruct struct {
 		nonExported string

vm/runtime/runtime.go

@@ -166,6 +166,11 @@ func Slice(array, from, to any) any {
 		if a > b {
 			a = b
 		}
+		if v.Kind() == reflect.Array && !v.CanAddr() {
+			newValue := reflect.New(v.Type()).Elem()
+			newValue.Set(v)
+			v = newValue
+		}
 		value := v.Slice(a, b)
 		if value.IsValid() {
 			return value.Interface()