KotlinReflectionParameterNameDiscoverer does not handle instance-extension methods properly [SPR-16119]

[spring-projects-issues]

Take Weiland opened SPR-16119 and commented

KotlinReflectionParameterNameDiscoverer filters out any parameters that are not of type KParameter.Kind.VALUE.
This works fine, except if you have an extension method inside a class, like this:

class Example {
    // this method will look like doSomething(String) in the bytecode and to Java
    fun String.doSomething()
}

The extension receiver (KParameter.Kind.EXTENSION_RECEIVER) parameter must not be ignored here, since as far as Java code is concerned, this is a regular parameter.

I noticed this problem in combination with Hibernate Validator, which will throw an ArrayIndexOutOfBoundsException when analyzing such a method.

I am in the process of creating a pull request to fix this issue and will update this ticket when it is submitted.

---

Affects: 5.0.1

Referenced from: commits 0e49e32

[spring-projects-issues]

Take Weiland commented

After further investigation, this is not limited to instance-extension methods but to all extension methods.
Extension receiver parameters must always be included in the parameter list and I have introduced the synthetic identifier $receiver for them.

I have submitted my pull request: #1578

[spring-projects-issues]

Sébastien Deleuze commented

Thanks for raising this issue and for your pull request. Indeed your Example class produces this Java code when I decompile produced bytecode in IDEA:

@Metadata(...)
public final class Example {
   @NotNull
   public final String doSomething(@NotNull String $receiver) {
      Intrinsics.checkParameterIsNotNull($receiver, "$receiver");
      return $receiver;
   }
}

That said, it seems to me you introduced $receiver both in implementation and test, which does not really check the actual parameter name. if I change your test to ```
assertThat(actualParams, is(arrayOf(method.parameters[0].name)))

```java.lang.AssertionError: Expected: is ["arg0"] but: was ["$receiver"]

Instead of introducing $receiver, what about adding a nullable method parameter to getParameterNames() and modify the implementation to use the actual parameter name with something like ```
String name = KParameter.Kind.EXTENSION_RECEIVER.equals(parameter.getKind()) ? method.getParameters()[i].getName() : parameter.getName();

Also any chance you could share a repro project with Hibernate Validator throwing an `ArrayIndexOutOfBoundsException`?

[spring-projects-issues]

Take Weiland commented

Yes, I could do that, however I do not think it makes much sense here.
KotlinReflectionParameterNameDiscoverer deals with Kotlin reflection only, if it does not find parameter names there, it completely bails out. However the Kotlin reflection code never provides a parameter name for receiver parameters, so we have to synthesize one. If we fall back to the Java parameters like you suggest, we would always get arg0 for the extension receiver parameter name, since Java reflection cannot see the Kotlin parameter names unless you explicitly enable that with a Kotlin compiler flag. If that compiler flag is enabled, the Kotlin compiler synthesizes the same name for the receiver: $receiver.

I have created a repository containing a test demonstrating the problem here.

[spring-projects-issues]

Sébastien Deleuze commented

Good point, let's keep the meaningful parameter name.