Don't do null checks in pattern matching if explicit nulls is enabled

Author: abeln

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

@@ -308,9 +308,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
 
   override def intersectUnrelatedAtomicTypes(tp1: Type, tp2: Type): Space = {
     // Precondition: !isSubType(tp1, tp2) && !isSubType(tp2, tp1).
-    if (ctx.explicitNulls) {
-      // No additional checks required: since subtyping already reflects nullability.
-    } else  if (tp1.isNullType || tp2.isNullType) {
+    if (!ctx.explicitNulls && (tp1.isNullType || tp2.isNullType)) {
       // Since projections of types don't include null, intersection with null is empty.
       return Empty
     }
@@ -791,20 +789,14 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
         }
 
         // if last case is `_` and only matches `null`, produce a warning
-        if (i == cases.length - 1 && !isNullLit(pat) ) {
+        // If explicit nulls are enabled, this check isn't needed because most of the cases
+        // that would trigger it would also trigger unreachability warnings.
+        if (!ctx.explicitNulls && i == cases.length - 1 && !isNullLit(pat) ) {
           val simpl = simplify(minus(covered, prevs))
-          if (ctx.explicitNulls) {
-            simpl match {
-              case space if isNullSpace(space) =>
-                ctx.warning(MatchCaseOnlyNullWarning(), pat.sourcePos)
-              case _ =>
-            }
-          } else {
-            simpl match {
-              case Typ(`constantNullType`, _) =>
-                ctx.warning(MatchCaseOnlyNullWarning(), pat.sourcePos)
-              case _ =>
-            }
+          simpl match {
+            case Typ(`constantNullType`, _) =>
+              ctx.warning(MatchCaseOnlyNullWarning(), pat.sourcePos)
+            case _ =>
           }
         }
       }

tests/explicit-nulls/neg-patmat/patmat1.scala

@@ -26,7 +26,7 @@ class Foo {
   a2 match {
     case Dog(_) => 100
     case Cat => 200
-    case _ => 300 // error: only matches null
+    case _ => 300
   }
 
   val a3: Animal|Null = ???