Only strip under unsafeNulls

[Cherry-picked ab240f1]

Author: dwijnand

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

@@ -808,12 +808,15 @@ object SpaceEngine {
     doShow(s)
   }
 
+  extension (self: Type) private def stripUnsafeNulls()(using Context): Type =
+    if Nullables.unsafeNullsEnabled then self.stripNull() else self
+
   private def exhaustivityCheckable(sel: Tree)(using Context): Boolean = trace(i"exhaustivityCheckable($sel ${sel.className})") {
     val seen = collection.mutable.Set.empty[Symbol]
 
     // Possible to check everything, but be compatible with scalac by default
     def isCheckable(tp: Type): Boolean = trace(i"isCheckable($tp ${tp.className})"):
-      val tpw = tp.widen.dealias.stripNull()
+      val tpw = tp.widen.dealias.stripUnsafeNulls()
       val classSym = tpw.classSymbol
       classSym.is(Sealed) && !tpw.isLargeGenericTuple || // exclude large generic tuples from exhaustivity
                                                          // requires an unknown number of changes to make work
@@ -859,7 +862,7 @@ object SpaceEngine {
   })
 
   def checkExhaustivity(m: Match)(using Context): Unit = trace(i"checkExhaustivity($m)") {
-    val selTyp = toUnderlying(m.selector.tpe.stripNull()).dealias
+    val selTyp = toUnderlying(m.selector.tpe.stripUnsafeNulls()).dealias
     val targetSpace = trace(i"targetSpace($selTyp)")(project(selTyp))
 
     val patternSpace = Or(m.cases.foldLeft(List.empty[Space]) { (acc, x) =>

tests/warn/i20132.stream-Tuple2.safeNulls.check

@@ -0,0 +1,8 @@
+-- [E029] Pattern Match Exhaustivity Warning: tests/warn/i20132.stream-Tuple2.safeNulls.scala:8:24 ---------------------
+8 |    xs.asJava.forEach { case (a, b) => // warn
+  |                        ^
+  |                        match may not be exhaustive.
+  |
+  |                        It would fail on pattern case: _: Null
+  |
+  | longer explanation available when compiling with `-explain`

tests/warn/i20132.stream-Tuple2.safeNulls.fixed.scala

@@ -0,0 +1,12 @@
+//> using options -Yexplicit-nulls -Yno-flexible-types
+
+import scala.jdk.CollectionConverters.*
+
+class Test2:
+  def t1: Unit = {
+    val xs = List.empty[(String, String)]
+    xs.asJava.forEach {
+      case (a, b) => ()
+      case null   => ()
+    }
+  }

tests/warn/i20132.stream-Tuple2.safeNulls.scala

@@ -0,0 +1,11 @@
+//> using options -Yexplicit-nulls -Yno-flexible-types
+
+import scala.jdk.CollectionConverters.*
+
+class Test2:
+  def t1: Unit = {
+    val xs = List.empty[(String, String)]
+    xs.asJava.forEach { case (a, b) => // warn
+      ()
+    }
+  }