Allow Kotlin Collections.List for @All qualifier

[Dudeplayz]

Describe the bug

I was trying to inject different implementations of the same interface at the same time. Following the docs I have done it with:

@Inject
@All
protected lateinit var processors: List<IProcessor>

But on compilation it throws:

Build step io.quarkus.arc.deployment.ArcProcessor#generateResources threw an exception: javax.enterprise.inject.spi.DefinitionException: Wildcard is not a legal type argument for ...Resource#processors

As I am using Kotlin, I changed the type of the list to MutableList, which works.
I think List is the more appropriate type following the docs: The injected instance is an immutable list of the contextual references of the disambiguated beans.. So either both or only List should be supported. The later one could break existing code.

I am also unsure, if the wildcard exception is the correct exception in this case?

Expected behavior

An immutable list is injected for List<T>.

Actual behavior

An exception is thrown.

How to Reproduce?

No response

Output of uname -a or ver

No response

Output of java -version

No response

GraalVM version (if different from Java)

No response

Quarkus version or git rev

2.8.0.Final

Build tool (ie. output of mvnw --version or gradlew --version)

mvn

Additional information

No response

[quarkus-bot]

/cc @evanchooly

[gsmet]

/cc @mkouba @manovotn

[mkouba]

It's not a bug but a problem caused by the fact that kotlin translates List<IProcessor> to something like java.util.List<? extends IProcessor>, i.e. the extracted required type is ? extends IProcessor and wildcard is an illegal injection point type. You can also replace List<IProcessor> with java.util.List<IProcessor> See more information in this comment .

[Dudeplayz]

Ok, but what is different for the MutableList? Is this a bug in Kotlin or wanted?

[Dudeplayz]

I have found this older ticket, stating to keep it as it is: https://youtrack.jetbrains.com/issue/KT-5792

[mkouba]

I don't think it's a bug in Kotlin. The point is that Kotlin's List is not java.util.List (it's read-only and does not translate 1:1 to java.util.List) and therefore you can't expect the @All qualifier to just work. Kotlin's MutableList on the other hand seems to behave like an alias for java.util.List.

@Ladicek and @evanchooly might have a better explanation ;-).

[Ladicek]

Right. Kotlin's List is read-only, so it can be and is covariant in its type parameter: interface List<out E>. Since Java/JVM only support use-site variance, usages of that type are represented in bytecode as List<? extends Whatever>. Which is an invalid injection point type.

On the other hand, Kotlin's MutableList is mutable, so it must be (and is) invariant: interface MutableList<E>. In bytecode, usages of that type look like MutableList<Whatever>. And that is a valid injection point type.

Technically, I think we could make a special case for this situation, but I'll leave that decision up to @mkouba.

[mkouba]

Technically, I think we could make a special case for this situation, but I'll leave that decision up to @mkouba.

Like relax the spec requirements for an injetion point type in this particular case? I'm not a big fan of similar things.

[kdubb]

I'm not sure how to achieve it but a special case for this situation would be greatly appreciated; every time I use @All I trip over this first haha.

[Dudeplayz]

I don't know how complex it is, but would it be possible to add a Kotlin compiler plugin (or something quarkus specific), which fixes such particular places?

Also, a mention in the docs somewhere that this is a problem would be helpful to reduce and avoid this pitfall.

[geoand]

The best we can do without having to go to great lengths is provide an actionable error message

[mkouba]

The best we can do without having to go to great lengths is provide an actionable error message

Hm, kotlin classes are annotated with a special annotation, right?

[Ladicek]

Yes, Kotlin-generated classes have the kotlin.Metadata annotation. ArC already uses it on one place (see SubclassGenerator).