How to nest template classes in Kotlin

Question

I'm trying to write some Kotlin code to encapsulate the idea of fields and elements in a field (with a view towards doing some work with finite fields).

I'd like the classes to be tightly coupled where possible (e.g. a field element belongs to a field), but am running into exceptions with the Kotlin compilation process. Later on, I'll want to do extension fields, so I'm trying to set up an abstract framework and create the simplest implementation of it right now.

I've narrowed my code down to this smallest example that causes the issue, and am running this on the most recent built (1.9.22) of Kotlin as of this post:

abstract class AbstractField<F: AbstractField<F, E>, E: AbstractField<F, E>.AbstractField<E>> {
    abstract val zero: E

    abstract inner class AbstractFieldElement<E: AbstractFieldElement<E>> {
        abstract operator fun plus(other: E): E
    }
}

// Implementation
class Fp(val prime: Int): AbstractField<Fp, Fp.EFp>() {
    override val zero = EFp(0)

    inner class EFp(val value: Int): AbstractFieldElement<EFp>() {
        override operator fun plus(other: EFp): EFp =
            EFp((value + other.value) % prime)

    override fun toString(): String =
        "$value (mod $prime)"
    }
}

fun main() {
    val field = Fp(13)
    val e1 = field.EFp(8)
    val e2 = field.EFp(10)
    println("$e1 + $e2 = ${e1 + e2}")
}

Trying to compile this is giving me a very long stack trace of a compiler issue about the intermediate representation. If anyone has any insight or suggestions, they would be greatly appreciated. I know I can separate the inner class out, but I would really rather not unless it's my only option.

org.jetbrains.kotlin.backend.common.BackendException: Backend Internal error: Exception during IR lowering
File being compiled: /Users/vorpal/dev/kotlin/field_bug/src/main/kotlin/Main.kt
The root cause java.lang.RuntimeException was thrown at: org.jetbrains.kotlin.backend.jvm.codegen.FunctionCodegen.generate(FunctionCodegen.kt:51)
    at org.jetbrains.kotlin.backend.common.CodegenUtil.reportBackendException(CodegenUtil.kt:253)
    at org.jetbrains.kotlin.backend.common.CodegenUtil.reportBackendException$default(CodegenUtil.kt:237)
    at org.jetbrains.kotlin.backend.common.phaser.PerformByIrFilePhase.invokeSequential(performByIrFile.kt:65)
    at org.jetbrains.kotlin.backend.common.phaser.PerformByIrFilePhase.invoke(performByIrFile.kt:52)
    at org.jetbrains.kotlin.backend.common.phaser.PerformByIrFilePhase.invoke(performByIrFile.kt:38)
    at org.jetbrains.kotlin.backend.common.phaser.NamedCompilerPhase.phaseBody(CompilerPhase.kt:147)
    at org.jetbrains.kotlin.backend.common.phaser.AbstractNamedCompilerPhase.invoke(CompilerPhase.kt:94)
    at org.jetbrains.kotlin.backend.common.phaser.CompositePhase.invoke(PhaseBuilders.kt:29)
    at org.jetbrains.kotlin.backend.common.phaser.CompositePhase.invoke(PhaseBuilders.kt:16)
    at org.jetbrains.kotlin.backend.common.phaser.NamedCompilerPhase.phaseBody(CompilerPhase.kt:147)
    at org.jetbrains.kotlin.backend.common.phaser.AbstractNamedCompilerPhase.invoke(CompilerPhase.kt:94)
    at org.jetbrains.kotlin.backend.common.phaser.CompilerPhaseKt.invokeToplevel(CompilerPhase.kt:43)
    at org.jetbrains.kotlin.backend.jvm.JvmIrCodegenFactory.invokeCodegen(JvmIrCodegenFactory.kt:361)
    at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.runCodegen(KotlinToJVMBytecodeCompiler.kt:347)
    at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.compileModules$cli(KotlinToJVMBytecodeCompiler.kt:122)
    at org.jetbrains.kotlin.cli.jvm.compiler.KotlinToJVMBytecodeCompiler.compileModules$cli$default(KotlinToJVMBytecodeCompiler.kt:43)
    at org.jetbrains.kotlin.cli.jvm.K2JVMCompiler.doExecute(K2JVMCompiler.kt:165)
    at org.jetbrains.kotlin.cli.jvm.K2JVMCompiler.doExecute(K2JVMCompiler.kt:50)
    at org.jetbrains.kotlin.cli.common.CLICompiler.execImpl(CLICompiler.kt:104)
    at org.jetbrains.kotlin.cli.common.CLICompiler.execImpl(CLICompiler.kt:48)
    at org.jetbrains.kotlin.cli.common.CLITool.exec(CLITool.kt:101)
    at org.jetbrains.kotlin.incremental.IncrementalJvmCompilerRunner.runCompiler(IncrementalJvmCompilerRunner.kt:463)
    at org.jetbrains.kotlin.incremental.IncrementalJvmCompilerRunner.runCompiler(IncrementalJvmCompilerRunner.kt:62)
    at org.jetbrains.kotlin.incremental.IncrementalCompilerRunner.doCompile(IncrementalCompilerRunner.kt:477)
    at org.jetbrains.kotlin.incremental.IncrementalCompilerRunner.compileImpl(IncrementalCompilerRunner.kt:400)
    at org.jetbrains.kotlin.incremental.IncrementalCompilerRunner.compileNonIncrementally(IncrementalCompilerRunner.kt:281)
    at org.jetbrains.kotlin.incremental.IncrementalCompilerRunner.compile(IncrementalCompilerRunner.kt:125)
    at org.jetbrains.kotlin.daemon.CompileServiceImplBase.execIncrementalCompiler(CompileServiceImpl.kt:657)
    at org.jetbrains.kotlin.daemon.CompileServiceImplBase.access$execIncrementalCompiler(CompileServiceImpl.kt:105)
    at org.jetbrains.kotlin.daemon.CompileServiceImpl.compile(CompileServiceImpl.kt:1624)
    at java.base/jdk.internal.reflect.DirectMethodHandleAccessor.invoke(DirectMethodHandleAccessor.java:104)
    at java.base/java.lang.reflect.Method.invoke(Method.java:578)
    at java.rmi/sun.rmi.server.UnicastServerRef.dispatch(UnicastServerRef.java:360)
    at java.rmi/sun.rmi.transport.Transport$1.run(Transport.java:200)
    at java.rmi/sun.rmi.transport.Transport$1.run(Transport.java:197)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:714)
    at java.rmi/sun.rmi.transport.Transport.serviceCall(Transport.java:196)
    at java.rmi/sun.rmi.transport.tcp.TCPTransport.handleMessages(TCPTransport.java:598)
    at java.rmi/sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run0(TCPTransport.java:844)
    at java.rmi/sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.lambda$run$0(TCPTransport.java:721)
    at java.base/java.security.AccessController.doPrivileged(AccessController.java:400)
    at java.rmi/sun.rmi.transport.tcp.TCPTransport$ConnectionHandler.run(TCPTransport.java:720)
    at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1144)
    at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:642)
    at java.base/java.lang.Thread.run(Thread.java:1623)
Caused by: java.lang.RuntimeException: Exception while generating code for:
FUN BRIDGE name:getZero visibility:public modality:OPEN <> ($this:org.example.Fp) returnType:org.example.AbstractField.AbstractFieldElement<*>
  $this: VALUE_PARAMETER INSTANCE_RECEIVER name:<this> type:org.example.Fp
  EXPRESSION_BODY
    TYPE_OP type=org.example.AbstractField.AbstractFieldElement<*, *, *> origin=IMPLICIT_CAST typeOperand=org.example.AbstractField.AbstractFieldElement<*, *, *>
      RETURN type=kotlin.Nothing from='public open fun getZero (): org.example.AbstractField.AbstractFieldElement<*> declared in org.example.Fp'
        CALL 'public open fun <get-zero> (): org.example.Fp.EFp declared in org.example.Fp' type=org.example.Fp.EFp origin=BRIDGE_DELEGATION
          $this: GET_VAR '<this>: org.example.Fp declared in org.example.Fp.getZero' type=org.example.Fp origin=null

    at org.jetbrains.kotlin.backend.jvm.codegen.FunctionCodegen.generate(FunctionCodegen.kt:51)
    at org.jetbrains.kotlin.backend.jvm.codegen.FunctionCodegen.generate$default(FunctionCodegen.kt:43)
    at org.jetbrains.kotlin.backend.jvm.codegen.ClassCodegen.generateMethodNode(ClassCodegen.kt:390)
    at org.jetbrains.kotlin.backend.jvm.codegen.ClassCodegen.generateMethod(ClassCodegen.kt:407)
    at org.jetbrains.kotlin.backend.jvm.codegen.ClassCodegen.generate(ClassCodegen.kt:169)
    at org.jetbrains.kotlin.backend.jvm.FileCodegen.lower(JvmPhases.kt:41)
    at org.jetbrains.kotlin.backend.common.phaser.FileLoweringPhaseAdapter.invoke(PhaseBuilders.kt:120)
    at org.jetbrains.kotlin.backend.common.phaser.FileLoweringPhaseAdapter.invoke(PhaseBuilders.kt:116)
    at org.jetbrains.kotlin.backend.common.phaser.NamedCompilerPhase.phaseBody(CompilerPhase.kt:147)
    at org.jetbrains.kotlin.backend.common.phaser.AbstractNamedCompilerPhase.invoke(CompilerPhase.kt:94)
    at org.jetbrains.kotlin.backend.common.phaser.PerformByIrFilePhase.invokeSequential(performByIrFile.kt:62)
    ... 42 more
Caused by: java.lang.AssertionError: -2 trailing arguments were found in this type: org.example.AbstractField.AbstractFieldElement<*>
    at org.jetbrains.kotlin.backend.jvm.mapping.IrTypeMappingKt.buildPossiblyInnerType(IrTypeMapping.kt:71)
    at org.jetbrains.kotlin.backend.jvm.mapping.IrTypeMappingKt.buildPossiblyInnerType(IrTypeMapping.kt:81)
    at org.jetbrains.kotlin.backend.jvm.mapping.IrTypeMappingKt.buildPossiblyInnerType(IrTypeMapping.kt:64)
    at org.jetbrains.kotlin.backend.jvm.mapping.IrTypeMapper.writeGenericType(IrTypeMapper.kt:148)
    at org.jetbrains.kotlin.backend.jvm.mapping.IrTypeMapper.writeGenericType(IrTypeMapper.kt:41)
    at org.jetbrains.kotlin.types.AbstractTypeMapper.mapClassType(AbstractTypeMapper.kt:192)
    at org.jetbrains.kotlin.types.AbstractTypeMapper.mapType(AbstractTypeMapper.kt:89)
    at org.jetbrains.kotlin.types.AbstractTypeMapper.mapType(AbstractTypeMapper.kt:53)
    at org.jetbrains.kotlin.backend.jvm.mapping.IrTypeMapper.mapType(IrTypeMapper.kt:134)
    at org.jetbrains.kotlin.backend.jvm.codegen.ClassCodegen$typeMapper$1.mapType(ClassCodegen.kt:101)
    at org.jetbrains.kotlin.backend.jvm.mapping.MethodSignatureMapper.mapReturnType(MethodSignatureMapper.kt:189)
    at org.jetbrains.kotlin.backend.jvm.mapping.MethodSignatureMapper.mapReturnType(MethodSignatureMapper.kt:169)
    at org.jetbrains.kotlin.backend.jvm.mapping.MethodSignatureMapper.mapSignature(MethodSignatureMapper.kt:277)
    at org.jetbrains.kotlin.backend.jvm.mapping.MethodSignatureMapper.mapSignature$default(MethodSignatureMapper.kt:227)
    at org.jetbrains.kotlin.backend.jvm.mapping.MethodSignatureMapper.mapSignatureWithGeneric(MethodSignatureMapper.kt:225)
    at org.jetbrains.kotlin.backend.jvm.codegen.FunctionCodegen.doGenerate(FunctionCodegen.kt:55)
    at org.jetbrains.kotlin.backend.jvm.codegen.FunctionCodegen.generate(FunctionCodegen.kt:47)
    ... 52 more