Fix isBottomType

Need to widen before testing

Author: odersky

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

@@ -1175,11 +1175,13 @@ class Definitions {
   def isBottomClassAfterErasure(cls: Symbol): Boolean = cls == NothingClass || cls == NullClass
 
   def isBottomType(tp: Type): Boolean =
-    if (ctx.explicitNulls && !ctx.phase.erasedTypes) tp.isCombinedRef(NothingClass)
-    else isBottomTypeAfterErasure(tp)
+    val tpw = tp.widen
+    tpw.isCombinedRef(NothingClass)
+    || tpw.isCombinedRef(NullClass) && (!ctx.explicitNulls || ctx.phase.erasedTypes)
 
   def isBottomTypeAfterErasure(tp: Type): Boolean =
-    tp.isCombinedRef(NothingClass) || tp.isCombinedRef(NullClass)
+    val tpw = tp.widen
+    tpw.isCombinedRef(NothingClass) || tpw.isCombinedRef(NullClass)
 
   /** Is a function class.
    *   - FunctionXXL

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

@@ -1539,9 +1539,6 @@ object Types {
     /** Is this (an alias of) the `scala.Null` type? */
     final def isNullType(using Context) = isRef(defn.NullClass)
 
-    /** Is this (an alias of) the `scala.Null` or `scala.Nothing` type? */
-    final def isNullOrNothingType(using Context) = isNullType || isRef(defn.NothingClass)
-
     /** The resultType of a LambdaType, or ExprType, the type itself for others */
     def resultType(using Context): Type = this