Compiler crash: assertion failed during "superaccessors" stage.

[scabug]

object Crash {
   implicit class Chain[T](val wrapped:T) extends AnyVal {
      def andThen[P,R](f: P => R)(implicit ev: T <:< (() => P)): R =
        f(ev(wrapped).apply)
   }

   class Foo {
      var x = 0
      def getX = x
   }

   def main(args: Seq[String]) {
      val result = new Foo {
        override def getX = {  x += 1 } andThen { _: Unit => x }
      }
      println(result)
   }
}

Compiling this causes compiler to crash:

Exception in thread "main" java.lang.AssertionError: assertion failed: 
  x.<$plus$eq: error>(1)
     while compiling: Crash.scala
        during phase: superaccessors
     library version: version 2.11.7
    compiler version: version 2.11.7
  reconstructed args: 

  last tree to typer: Ident(x)
       tree position: line 2 of Crash.scala
            tree tpe: Boolean
              symbol: value x
   symbol definition: x: Boolean (a TermSymbol)
      symbol package: <none>
       symbol owners: value x
           call site: method getX in package <empty>

== Source file context for tree position ==

     0 object Crash {
     1    implicit class Chain[T](val wrapped:T) extends AnyVal {
     2       def andThen[P,R](f: P => R)(implicit ev: T <:< (() => P)): R =
     3         f(ev(wrapped).apply)
     4    }
     5 
     6    class Foo {
	at scala.tools.nsc.typechecker.SuperAccessors$SuperAccTransformer.transform(SuperAccessors.scala:358)
	at scala.tools.nsc.typechecker.SuperAccessors$SuperAccTransformer.transform(SuperAccessors.scala:71)
	at scala.reflect.internal.Trees$class.itransform(Trees.scala:1345)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)
	at scala.reflect.internal.SymbolTable.itransform(SymbolTable.scala:16)

I understand, that this code is really messed up in more ways than one ... Still would be nice to get a more or less descriptive explanation of what I've done wrong rather than crashing.

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-9707?orig=1
Reporter: Dima (dimatkach-at-gmail.com)
Affected Versions: 2.11.7, 2.12.0-RC1

[scabug]

Dima (dimatkach-at-gmail.com) said:
Note: changing {code}getX{code} to look like this: {code}override def getX = { () => x = x + 1 } andThen { _:Unit => x }{code} fixes the crash.

[SethTisue]

% scala-cli compile -S 3.3.1-RC1 .
Compiling project (Scala 3.3.1-RC1, JVM)
[error] ./Crash.scala:12:32
[error] '=' expected, but '{' found
[error]    def main(args: Seq[String]) {
[error]                                ^
Error compiling project (Scala 3.3.1-RC1, JVM)
Compilation failed

[som-snytt]

It doesn't like the increment expression, which is rewritten. Less crashy:

object Crash {
   implicit class Chain[T](val wrapped:T) extends AnyVal {
      def andThen[P,R](f: P => R)(implicit ev: T <:< (() => P)): R =
        f(ev(wrapped).apply())
   }

   class Foo {
      var x = 0
      def getX = x
   }

   def main(args: Seq[String]): Unit = {
      val result = new Foo {
        override def getX = {  x += 1; () } andThen { (_: Unit) => x }
      }
      println(result)
   }
}

Scala 2

t9707.scala:15: error: Cannot prove that Unit <:< () => Unit.
        override def getX = {  x += 1; () } andThen { (_: Unit) => x }
                                            ^
1 error

Scala 3

-- [E172] Type Error: test/files/pos/t9707.scala:15:66 -----------------------------------------------------------------
15 |        override def getX = {  x += 1 } andThen { (_: Unit) => x }
   |                                                                  ^
   |                                                                  Cannot prove that Unit <:< () => Unit.
1 error found