Comment on the empty cases in notNullInfoFromCases.

[Cherry-picked 200c038]

Author: noti0na1

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

@@ -1868,9 +1868,11 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
   }
 
   private def notNullInfoFromCases(initInfo: NotNullInfo, cases: List[CaseDef])(using Context): NotNullInfo =
-    if cases.nonEmpty then
-      initInfo.seq(cases.map(_.notNullInfo).reduce(_.alt(_)))
-    else initInfo
+    if cases.isEmpty then
+      // Empty cases is not allowed for match tree in the source code,
+      // but it can be generated by inlining: `tests/pos/i19198.scala`.
+      initInfo
+    else cases.map(_.notNullInfo).reduce(_.alt(_))
 
   def typedCases(cases: List[untpd.CaseDef], sel: Tree, wideSelType: Type, pt: Type)(using Context): List[CaseDef] =
     var caseCtx = ctx