Rework ParamRef#underlying handling in disjointnessBoundary

Author: dwijnand

compiler/src/dotty/tools/dotc/core/TypeComparer.scala

@@ -2857,14 +2857,19 @@ class TypeComparer(@constructorOnly initctx: Context) extends ConstraintHandling
         tp
       case tp: HKTypeLambda =>
         tp
+      case tp: ParamRef =>
+        val st = tp.superTypeNormalized
+        if st.exists then
+          disjointnessBoundary(st)
+        else
+          // workaround for when ParamRef#underlying returns NoType
+          defn.AnyType
       case tp: TypeProxy =>
         disjointnessBoundary(tp.superTypeNormalized)
       case tp: WildcardType =>
         disjointnessBoundary(tp.effectiveBounds.hi)
       case tp: ErrorType =>
         defn.AnyType
-      case NoType => // ParamRef#superTypeNormalized can return NoType
-        defn.AnyType
     end disjointnessBoundary
 
     (disjointnessBoundary(tp1), disjointnessBoundary(tp2)) match