Fix bug in handling of nullable unions during widening

We need to call normalize on the widened type, as opposed
to the receiver of `widenUnion`, which need not be a union (could be a
TermRef).

Author: abeln

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

@@ -1194,10 +1194,10 @@ object Types {
      *  then the top-level union isn't widened. This is needed so that type inference can infer nullable types.
      */
     def widenUnion(implicit ctx: Context): Type = widen match {
-      case OrType(tp1, tp2) =>
-        if (ctx.explicitNulls && isNullableUnion) {
+      case tp @ OrType(tp1, tp2) =>
+        if (ctx.explicitNulls && tp.isNullableUnion) {
           // Don't widen `T|Null`, since otherwise we wouldn't be able to infer nullable unions.
-          val OrType(leftTpe, nullTpe) = normNullableUnion
+          val OrType(leftTpe, nullTpe) = tp.normNullableUnion
           OrType(leftTpe.widenUnion, nullTpe)
         } else {
           ctx.typeComparer.lub(tp1.widenUnion, tp2.widenUnion, canConstrain = true) match {

tests/explicit-nulls/pos/dont-widen-singleton.scala

@@ -0,0 +1,9 @@
+
+// Test that we correctly handle nullable unions when widening
+// (we don't widen them).
+class Test {
+  def foo(): Unit = {
+    val x: String|Null = ???
+    val y = x // this used to crash the compiler
+  }
+}