Compiler crash when `enum` class combine with `given` instance

[Eastsun]

minimized code

import scala.{Tuple => STuple}

enum Datatype[T] {
  case Tuple[T <: STuple](elems: STuple.Map[T, Datatype]) extends Datatype[T]
}

object Datatype {
  given [H, T <: STuple](using ht: Datatype[H], tt: Datatype[T]) as Datatype[H *: T] = tt match {
    case Datatype.Tuple(elems) => Datatype.Tuple(ht *: elems)
  }
}

Crash output (click arrow to expand)

[info] Compiling 1 Scala source to E:\dotty-hft\target\scala-0.22\classes ...
java.lang.StackOverflowError while compiling E:\dotty-hft\src\main\scala\Datatype.scala
[error] ## Exception when compiling 1 sources to E:\dotty-hft\target\scala-0.22\classes
[error] null
[error] dotty.tools.dotc.core.TypeOps$AbstractTypeMap$1.apply(TypeOps.scala:604)
[error] dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:4805)
[error] dotty.tools.dotc.core.TypeOps$AbstractTypeMap$1.apply(TypeOps.scala:604)
[error] dotty.tools.dotc.core.Types$TypeMap.op$4(Types.scala:4797)
[error] dotty.tools.dotc.core.Types$TypeMap.mapArgs$1(Types.scala:4797)
[error] dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:4805)
[error] dotty.tools.dotc.core.TypeOps$AbstractTypeMap$1.apply(TypeOps.scala:604)
[error] dotty.tools.dotc.core.Types$TypeMap.op$4(Types.scala:4797)
[error] dotty.tools.dotc.core.Types$TypeMap.mapArgs$1(Types.scala:4797)
[error] dotty.tools.dotc.core.Types$TypeMap.mapArgs$1(Types.scala:4799)
[error] dotty.tools.dotc.core.Types$TypeMap.mapOver(Types.scala:4805)
[error] dotty.tools.dotc.core.TypeOps$AbstractTypeMap$1.apply(TypeOps.scala:604)
...
[error] 	at dotty.tools.dotc.core.TypeOps$AbstractTypeMap$1.apply(TypeOps.scala:586)
[error] (Compile / compileIncremental) java.lang.StackOverflowError
[error] Total time: 3 s, completed 2020-2-19 18:23:41

The compiler also crash if change the enum to sealed trait or sealed abstract class , e.g.:

import scala.{Tuple => STuple}

sealed trait Datatype[T] 

object Datatype {
  case class Tuple[T <: STuple](elems: STuple.Map[T, Datatype]) extends Datatype[T]
  
  given [H, T <: STuple](using ht: Datatype[H], tt: Datatype[T]) as Datatype[H *: T] = tt match {
    case Datatype.Tuple(elems) => Datatype.Tuple(ht *: elems)
  }
}

However, if I move the sealed then everything works fine:

import scala.{Tuple => STuple}

trait Datatype[T] 

object Datatype {
  case class Tuple[T <: STuple](elems: STuple.Map[T, Datatype]) extends Datatype[T]
  
  given [H, T <: STuple](using ht: Datatype[H], tt: Datatype[T]) as Datatype[H *: T] = tt match {
    case Datatype.Tuple(elems) => Datatype.Tuple(ht *: elems)
  }
}

[anatoliykmetyuk]

Can be minimized to:

sealed trait Datatype[T]
case class Foo[T <: Tuple](elems: Tuple.Map[T, Datatype]) extends Datatype[T]

def g =
  val tt: Datatype[_] = ???
  tt match {
    case Foo(_) => ???
  }

The latest stack frame before the stack overflow loop frames start:

TypeOps.instantiateToSubType compiler/src/dotty/tools/dotc/core/TypeOps.scala:665

Loop trace, frames immediately from the start frame and till the first iteration of the loop finishes:

Trace

    Types$TypeMap.op$4 compiler/src/dotty/tools/dotc/core/Types.scala:4852
    Types$TypeMap.mapArgs$1 compiler/src/dotty/tools/dotc/core/Types.scala:4852
    Types$TypeMap.mapArgs$1 compiler/src/dotty/tools/dotc/core/Types.scala:4854
    Types$TypeMap.mapOver compiler/src/dotty/tools/dotc/core/Types.scala:4860
    TypeOps$AbstractTypeMap$1.apply compiler/src/dotty/tools/dotc/core/TypeOps.scala:605
    Types$TypeMap.op$4 compiler/src/dotty/tools/dotc/core/Types.scala:4852
    Types$TypeMap.mapArgs$1 compiler/src/dotty/tools/dotc/core/Types.scala:4852
    Types$TypeMap.mapArgs$1 compiler/src/dotty/tools/dotc/core/Types.scala:4854
    Types$TypeMap.mapOver compiler/src/dotty/tools/dotc/core/Types.scala:4860
    TypeOps$AbstractTypeMap$1.apply compiler/src/dotty/tools/dotc/core/TypeOps.scala:605
    Types$TypeMap.mapOverLambda$1 compiler/src/dotty/tools/dotc/core/Types.scala:4891
    Types$TypeMap.mapOver compiler/src/dotty/tools/dotc/core/Types.scala:4897
    TypeOps$AbstractTypeMap$1.apply compiler/src/dotty/tools/dotc/core/TypeOps.scala:605
    TypeOps$AbstractTypeMap$1.apply compiler/src/dotty/tools/dotc/core/TypeOps.scala:587
    List.mapConserve scala-library-2.13.1.jar/scala/collection/immutable/List.scala:444
    Types$TypeMap.mapOver compiler/src/dotty/tools/dotc/core/Types.scala:4917
    TypeOps$AbstractTypeMap$1.apply compiler/src/dotty/tools/dotc/core/TypeOps.scala:605
    Types$TypeMap.mapOverLambda$1 compiler/src/dotty/tools/dotc/core/Types.scala:4891
    Types$TypeMap.mapOver compiler/src/dotty/tools/dotc/core/Types.scala:4897
    TypeOps$AbstractTypeMap$1.apply compiler/src/dotty/tools/dotc/core/TypeOps.scala:605
    TypeOps$AbstractTypeMap$1.apply compiler/src/dotty/tools/dotc/core/TypeOps.scala:598

Locations as seen from commit 63343d2 .

Has something to do with the Map match type being compiled separately. The following file compiles:

object lib {
  type Map[Tup] <: Tuple = Tup match {
    case Unit => Unit
    case h *: t => Map[t]
  }
}

sealed trait Datatype
case class Foo[T <: Tuple](elems: lib.Map[T]) extends Datatype

def g =
  val tt: Datatype = ???
  tt match {
    case Foo(_) => ???
  }

However split this file in two as follows:

lib.scala

object lib {
  type Map[Tup] <: Tuple = Tup match {
    case Unit => Unit
    case h *: t => Map[t]
  }
}

iss_1.scala

sealed trait Datatype
case class Foo[T <: Tuple](elems: lib.Map[T]) extends Datatype

def g =
  val tt: Datatype = ???
  tt match {
    case Foo(_) => ???
  }

And compile them separately:

;dotty/dotc lib.scala;dotty/dotc -classpath . iss_1.scala

And the compiler will crash as described in the issue.