AdaptTypeError - foundType is Erroneous: <error>{def g: Unit} (in typer phase)

[scabug]

The following case causes an assertion failure in the latest 2.12 version.

Unlike #10003, version 2.11.8 won't crash for this case. It seems to be a recent regression.

$ scalac -version
Scala compiler version 2.12.0-20161028-204919-0666377 -- Copyright 2002-2016, LAMP/EPFL and Lightbend, Inc.


$ scalac abc.scala
error: java.lang.AssertionError: assertion failed:
  AdaptTypeError - foundType is Erroneous: <error>{def g: Unit}
     while compiling: abc.scala
        during phase: typer
     library version: version 2.12.0-20161028-204919-0666377
    compiler version: version 2.12.0-20161028-204919-0666377
  reconstructed args:

  last tree to typer: Select(New, <init>)
       tree position: line 3 of abc.scala
            tree tpe: ()<error>{def g: Unit}
              symbol: constructor $anon
   symbol definition: def <init>(): $anon (a MethodSymbol)
      symbol package: <empty>
       symbol owners: constructor $anon -> <$anon: <error>> -> value <local Bar> -> class Bar
           call site: class Bar in package <empty>

== Source file context for tree position ==

     0 class Bar {
     1   def f (x : { def g }) {}
     2   f (new Foo { def g })
     3 }
        at scala.tools.nsc.Global.assert(Global.scala:229)
        at scala.tools.nsc.typechecker.ContextErrors$TyperContextErrors$TyperErrorGen$.AdaptTypeError(ContextErrors.scala:216)
        at scala.tools.nsc.typechecker.Typers$Typer.adaptMismatchedSkolems$1(Typers.scala:1044)
        at scala.tools.nsc.typechecker.Typers$Typer.lastTry$1(Typers.scala:1055)
        at scala.tools.nsc.typechecker.Typers$Typer.adaptExprNotFunMode$1(Typers.scala:1110)
        at scala.tools.nsc.typechecker.Typers$Typer.vanillaAdapt$1(Typers.scala:1156)
        at scala.tools.nsc.typechecker.Typers$Typer.adapt(Typers.scala:1200)
        at scala.tools.nsc.typechecker.Typers$Typer.runTyper$1(Typers.scala:5519)
        at scala.tools.nsc.typechecker.Typers$Typer.typedInternal(Typers.scala:5535)

（...）


$ cat abc.scala
class Bar {
  def f (x : { def g }) {}
  f (new Foo { def g })
}

[scabug]

Imported From: https://issues.scala-lang.org/browse/SI-10015?orig=1
Reporter: Qirun Zhang (helloqirun)
Affected Versions: 2.12.0-RC2
See #10003