crash on compilable code with Trees$UnAssignedTypeException

[helloqirun]

scalac can compile it. But dotc crashes.

The callstack is partially similar to that of #1568. However, the latest dotc emits another "assertion failed: position error" for #1568 instead.

$ cat abc.scala

class A {
  implicit def compareComparables[T](x: T)(implicit ord: Ordering[T]) =
    new ord.Ops(x)
  class Bippy { def compare(y: Bippy) = util Random }
  () < ()
}

$ dotc abc.scala

-- Error: abc.scala ----------------------------------------------------------------------------------------------------
2 |  implicit def compareComparables[T](x: T)(implicit ord: Ordering[T]) =
  |  ^
  |  result type of implicit definition needs to be given explicitly
3 |    new ord.Ops(x)
-- [E008] Member Not Found Error: abc.scala ----------------------------------------------------------------------------
5 |  () < ()
  |  ^^
  |  value `<` is not a member of Unit

exception occurred while typechecking abc.scala

exception occurred while compiling abc.scala
Exception in thread "main" dotty.tools.dotc.ast.Trees$UnAssignedTypeException: type of Select(Tuple(List()),<) is not assigned
        at dotty.tools.dotc.ast.Trees$Tree.tpe(Trees.scala:97)
        at dotty.tools.dotc.ast.Trees$DenotingTree.denot(Trees.scala:236)
        at dotty.tools.dotc.ast.Trees$ProxyTree.denot(Trees.scala:254)
        at dotty.tools.dotc.ast.Trees$Tree.symbol(Trees.scala:148)
        at dotty.tools.dotc.typer.Typer$$anonfun$typedClassDef$1$$anonfun$apply$19.apply(Typer.scala:1246)
        at dotty.tools.dotc.typer.Typer$$anonfun$typedClassDef$1$$anonfun$apply$19.apply(Typer.scala:1246)
        at scala.collection.immutable.List.map(List.scala:277)
        at dotty.tools.dotc.typer.Typer$$anonfun$typedClassDef$1.apply(Typer.scala:1246)
        at dotty.tools.dotc.typer.Typer$$anonfun$typedClassDef$1.apply(Typer.scala:1191)
        at dotty.tools.dotc.util.Stats$.track(Stats.scala:35)
        at dotty.tools.dotc.typer.Typer.typedClassDef(Typer.scala:1191)
        at dotty.tools.dotc.typer.Typer.typedNamed$1(Typer.scala:1427)
        at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1475)
<snipped>

[liufengyun]

Thanks @helloqirun , all bug reports up to this one are confirmed. They are all very good reports, we appreciate much the reporting!