Fix #6247: Remove incorrect early check

The check relied on the assumption that `locked` is always contained in
`state.ownedVars`, but this can stop being true when an owned type
variable is garbage-collected.

Author: smarter

compiler/src/dotty/tools/dotc/typer/Inferencing.scala

@@ -406,8 +406,14 @@ trait Inferencing { this: Typer =>
    */
   def interpolateTypeVars(tree: Tree, pt: Type, locked: TypeVars)(implicit ctx: Context): tree.type = {
     val state = ctx.typerState
-    if (state.ownedVars.size > locked.size) {
-      val qualifying = state.ownedVars -- locked
+
+    // Note that some variables in `locked` might not be in `state.ownedVars`
+    // anymore if they've been garbage-collected, so we can't use
+    // `state.ownedVars.size > locked.size` as an early check to avoid computing
+    // `qualifying`.
+    val qualifying = state.ownedVars -- locked
+
+    if (qualifying.nonEmpty) {
       typr.println(i"interpolate $tree: ${tree.tpe.widen} in $state, owned vars = ${state.ownedVars.toList}%, %, previous = ${locked.toList}%, % / ${state.constraint}")
       val resultAlreadyConstrained =
         tree.isInstanceOf[Apply] || tree.tpe.isInstanceOf[MethodOrPoly]

compiler/src/dotty/tools/dotc/util/SimpleIdentitySet.scala

@@ -7,6 +7,7 @@ import collection.mutable.ListBuffer
  */
 abstract class SimpleIdentitySet[+Elem <: AnyRef] {
   def size: Int
+  def nonEmpty: Boolean = size > 0
   def + [E >: Elem <: AnyRef](x: E): SimpleIdentitySet[E]
   def - [E >: Elem <: AnyRef](x: E): SimpleIdentitySet[Elem]
   def contains[E >: Elem <: AnyRef](x: E): Boolean

tests/pos/i6247.scala

@@ -0,0 +1,10 @@
+class Foo {
+  implicit class NN[T](x:T|Null) {
+    def nn: T = x.asInstanceOf[T]
+  }
+
+  val x1: String|Null = null
+  x1.nn.length
+  val x2: String = x1.nn
+  x1.nn.length
+}