Types other than structs cannot be boxed. For example, a primitive type cannot be boxed:
{% include “docs/reference/fidl/language/error-catalog/label/_bad.md” %}
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/bad/fi-0193.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
To box a primitive, put it in a single-member struct instead:
{% include “docs/reference/fidl/language/error-catalog/label/_good.md” %}
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/good/fi-0193.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
Note that some types can be made optional via the use of the optional constraint. See the optionality guide, or the expandable below, for more information.
<<../../../../development/languages/fidl/widgets/_optionality.md>>
