address review comments

Author: olhotak

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

@@ -1873,10 +1873,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
     var alreadyStripped = false
     cases.mapconserve { cas =>
       val case1 = typedCase(cas, sel, wideSelType, pt)(using caseCtx)
+      caseCtx = Nullables.afterPatternContext(sel, case1.pat)
       if !alreadyStripped && Nullables.matchesNull(case1) then
         wideSelType = wideSelType.stripNull
         alreadyStripped = true
-      caseCtx = Nullables.afterPatternContext(sel, case1.pat)
       case1
     }
 

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

@@ -0,0 +1,15 @@
+// Test flow-typing when NotNullInfos are from cases
+
+object MatchTest {
+  def f6(s: String | Null): String = s match {
+    case s2 => s2 // error
+    case null => "other" // error
+    case s3 => s3
+  }
+
+  def f7(s: String | Null): String = s match {
+    case null => "other"
+    case null => "other" // error
+    case s3 => s3
+  }
+}

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

@@ -44,4 +44,15 @@ object MatchTest {
     case s2 => s2
     case s3 => s3
   }
+
+  def f6(s: String | Null): String = s match {
+    case s3: String => s3
+    case null => "other"
+    case s4 => s4
+  }
+
+  def f7(s: String | Null): String = s match {
+    case s2 => s2.nn
+    case s3 => s3
+  }
 }