Only a type using the struct layout can be boxed; union, vector, string, client_end, server_end, and zx.Handle must use the optional constraint instead:
{% include “docs/reference/fidl/language/error-catalog/label/_bad.md” %}
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/bad/fi-0171.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
Convert box<T> to T:optional to fix this issue:
{% include “docs/reference/fidl/language/error-catalog/label/_good.md” %}
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/good/fi-0171.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
Only FIDL types that can be made optional with no change to the wire shape are allowed to use the optional constraint. See the optionality guide, or the expandable below, for more information.
<<../../../../development/languages/fidl/widgets/_optionality.md>>
