Strip null on the scrutinee

Without that, we end up with either a flexible type or a `| Null`.

Author: dwijnand

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

@@ -843,7 +843,7 @@ object SpaceEngine {
   /** Return the underlying type of non-module, non-constant, non-enum case singleton types.
    *  Also widen ExprType to its result type, and rewrap any annotation wrappers.
    *  For example, with `val opt = None`, widen `opt.type` to `None.type`. */
-  def toUnderlying(tp: Type)(using Context): Type = trace(i"toUnderlying($tp)")(tp match {
+  def toUnderlying(tp: Type)(using Context): Type = trace(i"toUnderlying($tp ${tp.className})")(tp match {
     case _: ConstantType                            => tp
     case tp: TermRef if tp.symbol.is(Module)        => tp
     case tp: TermRef if tp.symbol.isAllOf(EnumCase) => tp
@@ -854,7 +854,7 @@ object SpaceEngine {
   })
 
   def checkExhaustivity(m: Match)(using Context): Unit = trace(i"checkExhaustivity($m)") {
-    val selTyp = toUnderlying(m.selector.tpe).dealias
+    val selTyp = toUnderlying(m.selector.tpe.stripNull()).dealias
     val targetSpace = trace(i"targetSpace($selTyp)")(project(selTyp))
 
     val patternSpace = Or(m.cases.foldLeft(List.empty[Space]) { (acc, x) =>

tests/warn/i20132.future-Left.scala

@@ -0,0 +1,13 @@
+//> using options -Yexplicit-nulls -Yno-flexible-types
+
+import scala.language.unsafeNulls
+
+import java.util.concurrent.CompletableFuture
+import scala.jdk.CollectionConverters._
+
+class Test1:
+  def m1 =
+    val fut: CompletableFuture[Either[String, List[String]]] = ???
+    fut.thenApply:
+      case Right(edits: List[String]) => edits.asJava
+      case Left(error: String) => throw new Exception(error)