Sdk: invalid_annotation_target hints are not shown when top-level variable is used as annotation

Created on 16 Oct 2020 · 2Comments · Source: dart-lang/sdk

It's common practice for an annotation without parameters to be defined as a private class which is exposed as a public top-level variable:

const myAnnotation = _MyAnnotation();

class _MyAnnotation() {
  const _MyAnnotation();
}

import 'my_annotation.dart';

@myAnnotation
class MyClass {}

The new @Target annotation is a fantastic idea for enforcing correct usage of annotations. Unfortunately, the hints produced by this annotation (invalid_annotation_target) only occur when the annotation is used as a constructor and not as a top-level variable:

import 'package:meta/meta_meta.dart';

const myAnnotation = MyAnnotation();

@Target({TargetKind.classType})
class MyAnnotation() {
  const MyAnnotation();
}

import 'my_annotation.dart';

@myAnnotation // Hint is *NOT* produced for this incorrect target.
void f() {}

@MyAnnotation()
// ^^^^^^^^^^
// The annotation 'MyAnnotation' can only be used on classes [invalid_annotation_target]
void g() {}

I'd expect the invalid_annotation_target hint to be produced, regardless of how the annotation is consumed.