Fix #4060 by marking ghost symbols as unstable

Author: Blaisorblade

Diff for: compiler/src/dotty/tools/dotc/core/SymDenotations.scala

@@ -600,7 +600,7 @@ object SymDenotations {
 
     /** Is this a denotation of a stable term (or an arbitrary type)? */
     final def isStable(implicit ctx: Context) =
-      isType || is(Stable) || !(is(UnstableValue) || info.isInstanceOf[ExprType])
+      !is(Ghost) && (isType || is(Stable) || !(is(UnstableValue) || info.isInstanceOf[ExprType]))
 
     /** Is this a "real" method? A real method is a method which is:
      *  - not an accessor

Diff for: tests/neg/i4060.scala

@@ -0,0 +1,22 @@
+class X { type R }
+class T(ghost val a: X)(val value: a.R) // error
+
+object App {
+  def coerce[U, V](u: U): V = {
+    trait X { type R >: U }
+    trait Y { type R = V }
+
+    class T[A <: X](ghost val a: A)(val value: a.R) // error
+
+    object O { lazy val x : Y & X = ??? } // warning
+
+    val a = new T[Y & X](O.x)(u)
+    a.value
+  }
+
+  def main(args: Array[String]): Unit = {
+    val x: Int = coerce[String, Int]("a")
+    println(x + 1)
+
+  }
+}