Newtypes from RFC-0052: Type aliasing and new types are not fully implementented and cannot be used yet:
{% include “docs/reference/fidl/language/error-catalog/label/_bad.md” %}
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/bad/fi-0062.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
In the meantime, you can achieve something similar by defining a struct with a single element:
{% include “docs/reference/fidl/language/error-catalog/label/_good.md” %}
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/good/fi-0062-a.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
Alternatively, you can define an alias, but note that unlike a newtype this provides no type safety (that is, it can be used interchangeably with its underlying type):
{% include “docs/reference/fidl/language/error-catalog/label/_good.md” %}
{% includecode gerrit_repo="fuchsia/fuchsia" gerrit_path="tools/fidl/fidlc/tests/fidl/good/fi-0062-b.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}