docs/reference/fidl/language/error-catalog/_fi-0124.md

## fi-0124: Custom attribute argument must be string or bool {:#fi-0124}

The arguments on user-defined FIDL attributes are limited to being string or
boolean types:

{% 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-0124.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
```

{% 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-0124.test.fidl" exclude_regexp="\/\/ (Copyright 20|Use of|found in).*" %}
```

Unlike [official attributes][docs-fidl-attributes], the schema of user defined
attributes is not known to the compiler. Because of this, it is impossible for
the compiler to deduce the type of any given numeric argument - is `2` an
`int8`, a `uint64`, or a `float32`? There's no way for the compiler to know.

A possible solution to this problem would be to implement a first-class
`numeric` type in the JSON IR for ambiguous cases like these. However, because
custom attribute arguments are the only known use case for this today, this
feature has not been prioritized.

[docs-fidl-attributes]: /docs/reference/fidl/language/attributes.md