add missing @unchecked for for patmat

Author: liufengyun

compiler/src/dotty/tools/dotc/ast/Desugar.scala

@@ -859,7 +859,7 @@ object desugar {
         case IdPattern(named, tpt) =>
           Function(derivedValDef(pat, named, tpt, EmptyTree, Modifiers(Param)) :: Nil, body)
         case _ =>
-          makeCaseLambda(CaseDef(pat, EmptyTree, body) :: Nil, unchecked = false)
+          makeCaseLambda(CaseDef(pat, EmptyTree, body) :: Nil)
       }
 
       /** If `pat` is not an Identifier, a Typed(Ident, _), or a Bind, wrap

tests/patmat/for.scala

@@ -2,4 +2,6 @@ object Test {
   def foo[A, B](l: List[(A, B)]): List[A] = {
     for ((a, b) <- l) yield a
   }
+
+  def bar(xs: List[(Int, List[Int])]): Unit = for ( (_, x :: y :: xs) <- xs) yield x
 }