Fix scala#4661: Missing unreachable case warnings

Author: abeln

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -294,16 +294,21 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
 
   override def intersectUnrelatedAtomicTypes(tp1: Type, tp2: Type) = {
     val and = AndType(tp1, tp2)
-    // Precondition: !(tp1 <:< tp2) && !(tp2 <:< tp1)
-    // Then, no leaf of the and-type tree `and` is a subtype of `and`.
-    val res = inhabited(and)
+    // Precondition: !isSubType(tp1, tp2) && !isSubType(tp2 <:< tp1)
+    if (tp1 == nullType || tp2 == nullType) {
+      // Since projections of types don't include null, intersection with null is empty.
+      Empty
+    } else {
+      // Then, no leaf of the and-type tree `and` is a subtype of `and`.
+      val res = inhabited(and)
 
-    debug.println(s"atomic intersection: ${and.show} = ${res}")
+      debug.println(s"atomic intersection: ${and.show} = ${res}")
 
-    if (!res) Empty
-    else if (tp1.isSingleton) Typ(tp1, true)
-    else if (tp2.isSingleton) Typ(tp2, true)
-    else Typ(and, true)
+      if (!res) Empty
+      else if (tp1.isSingleton) Typ(tp1, true)
+      else if (tp2.isSingleton) Typ(tp2, true)
+      else Typ(and, true)
+    }
   }
 
   /** Whether the extractor is irrefutable */