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” %}
{% 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” %}
{% 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, 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.
