java.lang.invoke.LambdaConversionException: Incorrect number of parameters for bridge signature

[danieldietrich]

There seems to be a general scala/scalac problem that does occur with Scala 2.12.7+ but not with Scala 2.12.4 for example. I also tested it with Scala 2.13.0-M5.

@2m originally reported it as possible Vavr bug: vavr-io/vavr#2337
I found a minimal example that only depends on standard Java 8 (or above).

// file test/A.java
package test;
public interface A extends B {
    @Override A ap();
    @Override default A test(B b) { return null; }
}

// file test/B.java
package test;
public interface B {
    B ap();
    default B test(B b) { return null; }
}

// file ./Main.scala
object Main {
  def main(args: Array[String]): Unit = {
    val f: test.A = () => null
  }
}

Running it

Tested with Oracle JDK 1.8.0_181

javac -cp . ./test/B.java && javac -cp . ./test/A.java && scalac -d out -classpath . Main.scala && scala -classpath out:. Main

Output

Exception in thread "main" java.lang.BootstrapMethodError: call site initialization exception
        at java.lang.invoke.CallSite.makeSite(CallSite.java:341)
        at java.lang.invoke.MethodHandleNatives.linkCallSiteImpl(MethodHandleNatives.java:307)
        at java.lang.invoke.MethodHandleNatives.linkCallSite(MethodHandleNatives.java:297)
        at Main$.main(Main.scala:3)
        at Main.main(Main.scala)
        at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
        at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
        at java.lang.reflect.Method.invoke(Method.java:498)
        at scala.reflect.internal.util.ScalaClassLoader.$anonfun$run$2(ScalaClassLoader.scala:99)
        at scala.reflect.internal.util.ScalaClassLoader.asContext(ScalaClassLoader.scala:33)
        at scala.reflect.internal.util.ScalaClassLoader.asContext$(ScalaClassLoader.scala:31)
        at scala.reflect.internal.util.ScalaClassLoader$URLClassLoader.asContext(ScalaClassLoader.scala:130)
        at scala.reflect.internal.util.ScalaClassLoader.run(ScalaClassLoader.scala:99)
        at scala.reflect.internal.util.ScalaClassLoader.run$(ScalaClassLoader.scala:91)
        at scala.reflect.internal.util.ScalaClassLoader$URLClassLoader.run(ScalaClassLoader.scala:130)
        at scala.tools.nsc.CommonRunner.run(ObjectRunner.scala:22)
        at scala.tools.nsc.CommonRunner.run$(ObjectRunner.scala:21)
        at scala.tools.nsc.ObjectRunner$.run(ObjectRunner.scala:37)
        at scala.tools.nsc.CommonRunner.runAndCatch(ObjectRunner.scala:28)
        at scala.tools.nsc.CommonRunner.runAndCatch$(ObjectRunner.scala:27)
        at scala.tools.nsc.ObjectRunner$.runAndCatch(ObjectRunner.scala:37)
        at scala.tools.nsc.MainGenericRunner.runTarget$1(MainGenericRunner.scala:63)
        at scala.tools.nsc.MainGenericRunner.run$1(MainGenericRunner.scala:84)
        at scala.tools.nsc.MainGenericRunner.process(MainGenericRunner.scala:95)
        at scala.tools.nsc.MainGenericRunner$.main(MainGenericRunner.scala:100)
        at scala.tools.nsc.MainGenericRunner.main(MainGenericRunner.scala)
Caused by: java.lang.invoke.LambdaConversionException: Incorrect number of parameters for bridge signature (B)B; incompatible with ()A
        at java.lang.invoke.AbstractValidatingLambdaMetafactory.validateMetafactoryArgs(AbstractValidatingLambdaMetafactory.java:206)
        at java.lang.invoke.LambdaMetafactory.altMetafactory(LambdaMetafactory.java:473)
        at java.lang.invoke.CallSite.makeSite(CallSite.java:325)
        ... 26 more

Notes

The example does run properly if we do one of the following modifications:

1. removing parameter of method test():

public interface A extends B {
    @Override A ap();
    @Override default A test() { return null; }
}
public interface B {
    B ap();
    default B test() { return null; }
}

2. OR returning the same type B test(B b):

public interface A extends B {
    @Override A ap();
    @Override default B test(B b) { return null; }
}
public interface B {
    B ap();
    default B test(B b) { return null; }
}

Thanks for looking into it!

[adriaanm]

Interesting! Playing around with the repro, trying to minimise a bit more still:

public interface Fun0 {
    String ap();

    default Fun0 test(Fun0 b) { return null; }
}

public interface Fun0Impl extends Fun0 {
    default Fun0Impl test(Fun0 b) { return null; }
}

object Test extends App {
  (() => null): Fun0Impl
}

[adriaanm]

Looks like the linked PR regressed here. The log says

[log delambdafy] will add SAM bridges for (() => Test.this.$anonfun$f$1()): List(method test)
<snip>
Caused by: java.lang.invoke.LambdaConversionException: Incorrect number of parameters for bridge signature (Fun0)Fun0; incompatible with ()String
	at java.lang.invoke.AbstractValidatingLambdaMetafactory.validateMetafactoryArgs(AbstractValidatingLambdaMetafactory.java:206)

the sam bridges are only intended for bridges for the actual sam method, whereas here test needs a bridge, which AbstractValidatingLambdaMetafactory is unprepared for

[adriaanm]

I proposed a fix scheduled for 2.12.9

[danieldietrich]

Thx, that was fast!

[lrytz]

Fixed in scala/scala#7668