Metadata does not accept runtimeType literals

[rmacnak-google]

Runtime type literals are const and should be accepted.

import 'dart:mirrors';
@object
class C {}
main(){
 reflectClass(C).metadata; //Fails
}

import 'dart:mirrors';
const foo = Object;
@foo
class C {}
main(){
 reflectClass(C).metadata; //Works
}

[DartBot]

This comment was originally written by @mhausner

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I suspect the VM does not treat a class name as a compile-time constant expression.

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Added Triaged label.

[gbracha]

I believe Ryan verified that it does. It just doesn't accept it as metadata.

[rmacnak-google]

It does, otherwise in the second example
const foo = Object;
would not work.

[DartBot]

This comment was originally written by @seaneagan

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Would it be more useful to have @­Object be shorthand for @­Object() ?

For example we could add messages to deprecated annotations:

class Deprecated {

  String _message();

  const Deprecated([this.message]);
}

And then I can omit the parentheses when I don't want to include a message as in Java (C# ?):

@deprecated
oldApi();

and when I do want a message:

@deprecated('Use [newApi] instead.');
oldApi();

[gbracha]

One problem with the proposal in comment 4 is that an identifier used in metadata means something different than in the rest of Dart. Object now means new Object().

Besides, we have real uses for using class names as metadata.

[DartBot]

This comment was originally written by @seaneagan

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@object() already means "const Object()" whereas in the rest of dart,
Object() means a function application.

Also, I can see it being a common source of errors, using @­Object when one
means @­Object() (due to familiarity with Java or C#).

I have to think those would be much less common, any examples? I'm worried
that with the current syntax people will design annotations like:

class SomeAnnotation {}

just so they can refer to it as @­SomeAnnotation, but then if they ever want
to make it parameterizable, it would be a breaking change, whereas if they
were able to use the same @­SomeAnnotation syntax with:

class SomeAnnotation {
  const SomeAnnotation();
}

then making it parameterizable would be a backwards compatible change.

If there are cases to use a runtimeType as a metadata value, maybe
assigning it to a variable and using that is sufficient:

const object = Object;

@object
...

[iposva-google]

Set owner to @rmacnak-google.
Added Accepted label.

[DartBot]

This comment was originally written by @seaneagan

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What are the use cases for this?

There are plenty of use cases and demand for treating @­Foo as @­Foo(), as elaborated in issue #2.

[DartBot]

This comment was originally written by [email protected]

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This point was also discussed on the Dart misc list : https://groups.google.com/a/dartlang.org/d/msg/misc/bZ5kVTOnUN8/ZlQeTWpMnMwJ

[rmacnak-google]

This issue causes mirrors/immutable_collections_test to fail on Dartium.

[DartBot]

This comment was originally written by @seaneagan

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The analyzer also does not accept @­Foo (where Foo is a class), and even has a specific error message for it:

"Annotation creation must have arguments"

Maybe the best thing to do for 1.0 is to just disallow @­Foo, and figure out later whether to fix issue #2 or this issue?

[DartBot]

This comment was originally written by [email protected]

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If you do that, you will have an annotation nightmare to fix post 1.0 ...

I understand 1.0 is approaching, but please have a look to current annotations status in the different frameworks (Google and thirdparty), nobody is happy with current implementation.

[rmacnak-google]

Subsumed by Issue #14647.

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Added AsDesigned label.

[DartBot]

This comment was originally written by [email protected]

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As a Dart GDE, I am one of the strongest Dart fan, but I still think that annotations in Dart have a big problem and lead to ugly code.

Look at what AngularDart did with their recent support of injectable classes. We must use @­inject or @­Injectable().

Being able to use @­Injectable or @­Inject would have been so much better ...

[DartBot]

This comment was originally written by @seaneagan

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Sebastian, please star/comment issue #13582.