AbstractMethodError using Scala 2 http4s library/withDottyCompat

[rparree]

Hi,

I'm trying to create a small service using http4s (0.21) on Scala 3, using withDottyCompat
Using 3.0.0-M3 (same results with M2), sbt-dotty@0.5.0

A very much stripped down version is available here: https://github.com/rparree/http4s-dotty

When running i'm getting the following error:

Receiver class LivenessRoutes$$anon$1 does not define or inherit an implementation of the resolved method 'abstract void org$http4s$dsl$impl$Methods$_setter_$GET_$eq(org.http4s.Method$Semantics$Safe)' of interface org.http4s.dsl.impl.Methods.

See full stacktrace here: https://gist.github.com/rparree/e19d5ff29c885160c17c18ac5bdc017b#file-rte

After posting this on the http4s gitter channel, @rossabaker suggested that this could potentially be a withDottyCompat related problem.

[griggt]

Thanks for reporting. A small reproducer is:

// build.sbt
scalaVersion := "3.0.0-M3"
libraryDependencies += ("org.http4s" %% "http4s-dsl" % "0.21.13").withDottyCompat(scalaVersion.value)

// Main.scala
import org.http4s.dsl.Http4sDsl

trait F[x]

object Test {
  def main(args: Array[String]): Unit = {
    val dsl = new Http4sDsl[F] {}
  }
}

sbt> run
[info] compiling 1 Scala source to /tmp/i10827/target/scala-3.0.0-M3/classes ...
[info] done compiling
[info] running Test
[error] (run-main-0) java.lang.AbstractMethodError: Method Test$$anon$1.org$http4s$dsl$impl$Methods$_setter_$GET_$eq(Lorg/http4s/Method$Semantics$Safe;)V is abstract
[error] java.lang.AbstractMethodError: Method Test$$anon$1.org$http4s$dsl$impl$Methods$_setter_$GET_$eq(Lorg/http4s/Method$Semantics$Safe;)V is abstract
[error]         at Test$$anon$1.org$http4s$dsl$impl$Methods$_setter_$GET_$eq(Main.scala)
[error]         at org.http4s.dsl.impl.Methods.$init$(Methods.scala:12)
[error]         at Test$$anon$1.<init>(Main.scala:7)
[error]         at Test$.main(Main.scala:7)
[error]         at Test.main(Main.scala)
[error]         at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error]         at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[error]         at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error]         at java.lang.reflect.Method.invoke(Method.java:498)

I'm working on minimizing to a standalone example.

[griggt]

Standalone example:

First, a library compiled with Scala 2:

// build.sbt
scalaVersion := "2.13.4"
organization := "test.http4s"
name := "i10827"
version := "0.1.0-SNAPSHOT"

// Lib.scala
package test.http4s

class Method(name: String)

object Method {
  trait Safe
  trait PermitsBody extends Method

  type SafeMethodWithBody = Method with Safe with PermitsBody
  val GET: SafeMethodWithBody = new Method("GET") with Safe with PermitsBody
}

trait Methods {
  val GET: Method.GET.type = Method.GET
}

Then a test program consuming the library with Scala 3:

// build.sbt
scalaVersion := "3.0.0-M3"
libraryDependencies += ("test.http4s" %% "i10827" % "0.1.0-SNAPSHOT").withDottyCompat(scalaVersion.value)

// Main.scala
import test.http4s.Methods

object Test {
  def main(args: Array[String]): Unit = {
    val m = new Methods {}
  }
}

sbt:test> run
[info] compiling 1 Scala source to /src/dotty-issues/i10827/test/target/scala-3.0.0-M3/classes ...
[info] done compiling
[info] running Test
[error] (run-main-0) java.lang.AbstractMethodError: Test$$anon$1.test$http4s$Methods$_setter_$GET_$eq(Ltest/http4s/Method$Safe;)V
[error] java.lang.AbstractMethodError: Test$$anon$1.test$http4s$Methods$_setter_$GET_$eq(Ltest/http4s/Method$Safe;)V
[error]         at test.http4s.Methods.$init$(Lib.scala:14)
[error]         at Test$$anon$1.<init>(Main.scala:5)
[error]         at Test$.main(Main.scala:5)
[error]         at Test.main(Main.scala)
[error]         at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
[error]         at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62)
[error]         at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43)
[error]         at java.lang.reflect.Method.invoke(Method.java:498)

The generated bytecode (from 3.0.0-M3):

Compiled from "Main.scala"
final class Test$$anon$1 implements test.http4s.Methods {
  private test.http4s.Method$PermitsBody GET;

  public Test$$anon$1();
    Code:
       0: aload_0
       1: invokespecial #20                 // Method java/lang/Object."<init>":()V
       4: aload_0
       5: invokestatic  #24                 // InterfaceMethod test/http4s/Methods.$init$:(Ltest/http4s/Methods;)V
       8: invokestatic  #29                 // Method scala/runtime/Statics.releaseFence:()V
      11: return

  public test.http4s.Method$PermitsBody GET();
    Code:
       0: aload_0
       1: getfield      #34                 // Field GET:Ltest/http4s/Method$PermitsBody;
       4: areturn

  public void test$http4s$Methods$_setter_$GET_$eq(test.http4s.Method$PermitsBody);
    Code:
       0: aload_0
       1: aload_1
       2: putfield      #34                 // Field GET:Ltest/http4s/Method$PermitsBody;
       5: return
}

compared to 2.13.4:

Compiled from "Main.scala"
public final class Test$$anon$1 implements test.http4s.Methods {
  private test.http4s.Method$Safe GET;

  public test.http4s.Method$Safe GET();
    Code:
       0: aload_0
       1: getfield      #22                 // Field GET:Ltest/http4s/Method$Safe;
       4: areturn

  public void test$http4s$Methods$_setter_$GET_$eq(test.http4s.Method$Safe);
    Code:
       0: aload_0
       1: aload_1
       2: putfield      #22                 // Field GET:Ltest/http4s/Method$Safe;
       5: return

  public Test$$anon$1();
    Code:
       0: aload_0
       1: invokespecial #31                 // Method java/lang/Object."<init>":()V
       4: aload_0
       5: invokestatic  #35                 // InterfaceMethod test/http4s/Methods.$init$:(Ltest/http4s/Methods;)V
       8: invokestatic  #40                 // Method scala/runtime/Statics.releaseFence:()V
      11: return
}

See also #4619