add comment to code change

Author: liufengyun

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

@@ -1300,6 +1300,8 @@ class Typer extends Namer
       case _ =>
         if (tree.name == nme.WILDCARD) body1
         else {
+          // for a singleton pattern like `x @ Nil`, `x` should get the type from the scrutinee
+          // see tests/neg/3200b.scala and SI-1503
           val symTp =
             if (body1.tpe.isInstanceOf[TermRef]) pt1
             else body1.tpe.underlyingIfRepeated(isJava = false)