Stack overflows on a structural type definition

[marcin-chwedczuk]

reproduction steps

% scala --version
Scala code runner version 2.13.3 -- Copyright 2002-2020, LAMP/EPFL and Lightbend, Inc.

macOS, scala installed via brew install scala.

scala>   type MonadLike[A] = {
     |     def map[B](f: A => B): MonadLike[B]
     |   }
java.lang.StackOverflowError
        at scala.reflect.internal.Types$TypeRef.mapOver(Types.scala:2361)
        at scala.reflect.internal.tpe.TypeMaps$SubstMap.apply(TypeMaps.scala:728)
        at scala.reflect.internal.tpe.TypeMaps$SubstMap.apply(TypeMaps.scala:666)
        at scala.reflect.internal.Types$TypeRef.mapOver(Types.scala:2370)
        at scala.reflect.internal.tpe.TypeMaps$SubstMap.apply(TypeMaps.scala:728)
        at scala.reflect.internal.tpe.TypeMaps$TypeMap.applyToSymbolInfo(TypeMaps.scala:123)
        at scala.reflect.internal.tpe.TypeMaps$TypeMap.loop$1(TypeMaps.scala:117)
        at scala.reflect.internal.tpe.TypeMaps$TypeMap.firstChangedSymbol(TypeMaps.scala:121)
        at scala.reflect.internal.tpe.TypeMaps$TypeMap.mapOver(TypeMaps.scala:135)
        at scala.reflect.internal.Types$MethodType.mapOver(Types.scala:2925)
        at scala.reflect.internal.tpe.TypeMaps$SubstMap.apply(TypeMaps.scala:728)
        at scala.reflect.internal.Types$PolyType.mapOver(Types.scala:3029)

problem

A proper error should be displayed instead of SOE.

[som-snytt]

class C {
  type M[A] = scala.AnyRef {
    def m[B](f: _root_.scala.Function1[A, B]): M[B]
  }
}

And the doti says:

-- [E117] Syntax Error: t12151.scala:4:8 ---------------------------------------
4 |    def m[B](f: _root_.scala.Function1[A, B]): M[B]
  |    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  |Polymorphic refinement method m without matching type in parent type AnyRef is no longer allowed

Explanation
===========
Polymorphic method m is not allowed in the structural refinement of type AnyRef because
method m does not override any method in type AnyRef. Structural refinement does not allow for
polymorphic methods.

-- [E140] Cyclic Error: t12151.scala:3:7 ---------------------------------------
3 |  type M[A] = scala.AnyRef {
  |       ^
  |illegal cyclic type reference: alias ... of type M refers back to the type itself
2 errors found

[Jasper-M]

This seems to be a regression in 2.13.1.

2.13.0 still emits the illegal cyclic reference error:

scala> type MonadLike[A] = {
     |   def map[B](f: A => B): MonadLike[B]
     | }
         def map[B](f: A => B): MonadLike[B]
                                ^
On line 2: error: illegal cyclic reference involving method map