Fix #8083: Add test case

Author: odersky

tests/pos/i8083.scala

@@ -0,0 +1,26 @@
+trait A {
+  class B
+}
+abstract class Test2 { val a: A; val b: a.B }
+object Test2 {
+  def unapply(that: Test2): Option[(that.a.type, that.a.B)] = Some((that.a, that.b))
+}
+object anA extends A
+object Test extends App {
+  val t: Test2 { val a: anA.type; val b: anA.B } = new Test2 { val a: anA.type = anA; val b = new a.B }
+  t match {
+    case Test2(u, v) =>
+      u: A
+      u: t.a.type // error
+      v: t.a.B // error
+  }
+}
+object Test1 extends App {
+  object t extends Test2 { val a = anA; val b = new a.B }
+  t match {
+    case Test2(u, v) =>
+      u: A
+      u: t.a.type // error
+      v: t.a.B // error
+  }
+}