Opaque types not allowed in a class

[panacekcz]

Although all the examples of opaque types define them in objects, I didn't see any mention in the proposal of opaque types being intentionally disallowed in classes, so I guess it is an issue of the implementation.

Test code:

class C {
  opaque type T = Int
}

Output:

-- Error: opaque-class.scala:2:2 -----------------------------------
2 |  opaque type T = Int
  |  ^
  |type T in object T cannot be accessed as a member of C.this.T.type from class C.
one error found

[b-studios]

As of 0.17.0-bin-20190623-49c4a12-NIGHTLY your example crashes the compiler with:

[error] scala.MatchError: NoType (of class dotty.tools.dotc.core.Types$NoType$)
[error] 	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType(TreePickler.scala:270)
[error] 	at dotty.tools.dotc.core.tasty.TreePickler.pickleType(TreePickler.scala:144)
[error] 	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType$$anonfun$4(TreePickler.scala:226)
[error] 	at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
[error] 	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:44)
[error] 	at dotty.tools.dotc.core.tasty.TreePickler.pickleNewType(TreePickler.scala:228)
[error] 	at dotty.tools.dotc.core.tasty.TreePickler.pickleType(TreePickler.scala:144)
[error] 	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree$$anonfun$25(TreePickler.scala:529)
[error] 	at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
[error] 	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:44)
[error] 	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:533)
[error] 	at dotty.tools.dotc.core.tasty.TreePickler.pickleDef$$anonfun$2(TreePickler.scala:307)
[error] 	at dotty.runtime.function.JFunction0$mcV$sp.apply(JFunction0$mcV$sp.java:12)
[error] 	at dotty.tools.dotc.core.tasty.TreePickler.withLength(TreePickler.scala:44)
[error] 	at dotty.tools.dotc.core.tasty.TreePickler.pickleDef(TreePickler.scala:312)
[error] 	at dotty.tools.dotc.core.tasty.TreePickler.pickleTree(TreePickler.scala:508)
[error] 	at dotty.tools.dotc.core.tasty.TreePickler.pickleStats$$anonfun$2(TreePickler.scala:331)
[error] 	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:15)
[error] 	at dotty.runtime.function.JProcedure1.apply(JProcedure1.java:10)
[error] 	at scala.collection.immutable.List.foreach(List.scala:392)

[allanrenucci]

Regression test added in #6956