Fix panic traversing marked list

We need to unmark the result of HasIndex in order to compare it with
true or false without a panic.

ops.go

@@ -76,7 +76,10 @@ func Index(collection, key cty.Value, srcRange *Range) (cty.Value, Diagnostics)
  }
  }
 
- has := collection.HasIndex(key)
+ // Here we drop marks from HasIndex result, in order to allow basic
+ // traversal of a marked list, tuple, or map in the same way we can
+ // traverse a marked object
+ has, _ := collection.HasIndex(key).Unmark()
  if !has.IsKnown() {
  if ty.IsTupleType() {
  return cty.DynamicVal, nil

ops_test.go

@@ -40,6 +40,14 @@ func TestApplyPath(t *testing.T) {
  cty.StringVal("hello"),
  ``,
  },
+ {
+ cty.ListVal([]cty.Value{
+ cty.StringVal("hello"),
+ }).Mark("x"),
+ (cty.Path)(nil).Index(cty.NumberIntVal(0)),
+ cty.StringVal("hello").Mark("x"),
+ ``,
+ },
  {
  cty.TupleVal([]cty.Value{
  cty.StringVal("hello"),