fix #11967: flow typing nullability in pattern matches

compiler/src/dotty/tools/dotc/typer/Nullables.scala

@@ -190,6 +190,16 @@ object Nullables:
     // TODO: Add constant pattern if the constant type is not nullable
     case _ => false
 
+  def matchesNull(cdef: CaseDef)(using Context): Boolean =
+    cdef.guard.isEmpty && patMatchesNull(cdef.pat)
+
+  private def patMatchesNull(pat: Tree)(using Context): Boolean = pat match
+    case Literal(Constant(null)) => true
+    case Bind(_, pat) => patMatchesNull(pat)
+    case Alternative(trees) => trees.exists(patMatchesNull)
+    case _ if isVarPattern(pat) => true
+    case _ => false
+
   extension (infos: List[NotNullInfo])
 
     /** Do the current not-null infos imply that `ref` is not null?

compiler/src/dotty/tools/dotc/typer/Typer.scala

@@ -1843,12 +1843,17 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
   /** Special typing of Match tree when the expected type is a MatchType,
    *  and the patterns of the Match tree and the MatchType correspond.
    */
-  def typedDependentMatchFinish(tree: untpd.Match, sel: Tree, wideSelType: Type, cases: List[untpd.CaseDef], pt: MatchType)(using Context): Tree = {
+  def typedDependentMatchFinish(tree: untpd.Match, sel: Tree, wideSelType0: Type, cases: List[untpd.CaseDef], pt: MatchType)(using Context): Tree = {
     var caseCtx = ctx
+    var wideSelType = wideSelType0
+    var alreadyStripped = false
     val cases1 = tree.cases.zip(pt.cases)
       .map { case (cas, tpe) =>
         val case1 = typedCase(cas, sel, wideSelType, tpe)(using caseCtx)
         caseCtx = Nullables.afterPatternContext(sel, case1.pat)
+        if !alreadyStripped && Nullables.matchesNull(case1) then
+          wideSelType = wideSelType.stripNull
+          alreadyStripped = true
         case1
       }
       .asInstanceOf[List[CaseDef]]
@@ -1862,10 +1867,15 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     assignType(cpy.Match(tree)(sel, cases1), sel, cases1)
   }
 
-  def typedCases(cases: List[untpd.CaseDef], sel: Tree, wideSelType: Type, pt: Type)(using Context): List[CaseDef] =
+  def typedCases(cases: List[untpd.CaseDef], sel: Tree, wideSelType0: Type, pt: Type)(using Context): List[CaseDef] =
     var caseCtx = ctx
+    var wideSelType = wideSelType0
+    var alreadyStripped = false
     cases.mapconserve { cas =>
       val case1 = typedCase(cas, sel, wideSelType, pt)(using caseCtx)
+      if !alreadyStripped && Nullables.matchesNull(case1) then
+        wideSelType = wideSelType.stripNull
+        alreadyStripped = true
       caseCtx = Nullables.afterPatternContext(sel, case1.pat)
       case1
     }

tests/explicit-nulls/pos/flow-match.scala

@@ -12,4 +12,36 @@ object MatchTest {
     // after the null case, s becomes non-nullable
     case _ => s
   }
+
+  def f(s: String | Null): String = s match {
+    case null => "other"
+    case s2 => s2
+    case s3 => s3
+  }
+
+  class Foo
+
+  def f2(s: String | Null): String = s match {
+    case n @ null => "other"
+    case s2 => s2
+    case s3 => s3
+  }
+
+  def f3(s: String | Null): String = s match {
+    case null | "foo" => "other"
+    case s2 => s2
+    case s3 => s3
+  }
+
+  def f4(s: String | Null): String = s match {
+    case _ => "other"
+    case s2 => s2
+    case s3 => s3
+  }
+
+  def f5(s: String | Null): String = s match {
+    case x => "other"
+    case s2 => s2
+    case s3 => s3
+  }
 }