crash on compilable code: NullPointerException

[helloqirun]

scalac compiles but dotc crashes.

$ cat abc.scala

class DuplicateClassName { () => {
  { () =>
    }
}
}

$ dotc abc.scala

-- Error: abc.scala ----------------------------------------------------------------------------------------------------
1 |class DuplicateClassName { () => {
  |                           ^^
  |                           not a legal self type clause

exception occurred while typechecking abc.scala

exception occurred while compiling abc.scala
Exception in thread "main" java.lang.NullPointerException
	at dotty.tools.dotc.ast.tpd$.ref(tpd.scala:321)
	at dotty.tools.dotc.ast.untpd$.ref(untpd.scala:319)
	at dotty.tools.dotc.ast.desugar$.apply(Desugar.scala:936)
	at dotty.tools.dotc.typer.Typer.typedUnnamed$1(Typer.scala:1478)
	at dotty.tools.dotc.typer.Typer.typedUnadapted(Typer.scala:1484)
	at dotty.tools.dotc.typer.Typer$$anonfun$typed$2.apply(Typer.scala:1494)
	at dotty.tools.dotc.typer.Typer$$anonfun$typed$2.apply(Typer.scala:1492)
	at dotty.tools.dotc.reporting.Reporting$class.traceIndented(Reporter.scala:132)
	at dotty.tools.dotc.core.Contexts$Context.traceIndented(Contexts.scala:57)
<snipped>

[helloqirun]

A more clean code example:

class Foo { () => 0 }

[liufengyun]

Thanks for reporting @helloqirun .

This issue is related to #1683 and #1424 . We've decided to treat the code as a syntactical error, as @odersky commented here:

I think we should not try to overengineer fixing this. The scalac code doing this is quite impenetrable. I think "not a legal self type clause" is perfectly fine as an error message.

[liufengyun]

Reopen, as we need to fix the crash.