docs/reference/fidl/language/error-catalog/_fi-0160.md

## fi-0160: Type cannot be marked as optional twice {:#fi-0160}

This error occurs when a type is made optional twice. Usually,
this occurs when the type is marked optional at both its use
and declaration sites.

{% include "docs/reference/fidl/language/error-catalog/label/_bad.md" %}

```fidl
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/bad/fi-0160.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
```

To fix this error, only make the type optional once.

For example, you can remove `:optional` from the use site.

{% include "docs/reference/fidl/language/error-catalog/label/_good.md" %}

```fidl
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/good/fi-0160-a.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
```

You could also remove `:optional` from the alias declaration.

```fidl
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/good/fi-0160-b.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
```