2.12.4 regression: the compiler fails to reject unstable paths in types #10619
Description
With 2.12.3, and any other published version of Scala (incl. 2.11.12 and 2.13.0-M2):
$ ~/opt/scala-2.12.3/bin/scala
Welcome to Scala 2.12.3 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_40).
Type in expressions for evaluation. Or try :help.
scala> class Outer { class Inner }
defined class Outer
scala> def newOuter = new Outer
newOuter: Outer
scala> identity[newOuter.Inner] _
<console>:13: error: stable identifier required, but newOuter found.
identity[newOuter.Inner] _
^
scala> val x: newOuter.Inner = ???
<console>:12: error: stable identifier required, but newOuter found.
val x: newOuter.Inner = ???which is the correct behavior.
With 2.12.4:
$ ~/opt/scala-2.12.4/bin/scala
Welcome to Scala 2.12.4 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_40).
Type in expressions for evaluation. Or try :help.
scala> class Outer { class Inner }
defined class Outer
scala> def newOuter = new Outer
newOuter: Outer
scala> identity[newOuter.Inner] _
res0: Outer#Inner => Outer#Inner = $$Lambda$1060/2027587999@e7b265e
scala> val x: newOuter.Inner = ???
scala.NotImplementedError: an implementation is missing
at scala.Predef$.$qmark$qmark$qmark(Predef.scala:284)
... 29 elidedwhich is bogus.