Add flag to disable withFilter pattern desugaring

src/compiler/scala/tools/nsc/settings/ScalaSettings.scala

@@ -217,6 +217,11 @@ trait ScalaSettings extends AbsScalaSettings
 
   private def removalIn212 = "This flag is scheduled for removal in 2.12. If you have a case where you need this flag then please report a bug."
 
+  /**
+   * -Z Typelevel settings
+   */
+  val ZirrefutableGeneratorPatterns = BooleanSetting("-Zirrefutable-generator-patterns", "Treat patterns in for comprehensions as irrefutable. Do not add filter or withFilter calls.")
+
   /** Area-specific debug output.
    */
   val Ydocdebug               = BooleanSetting("-Ydoc-debug", "Trace all scaladoc activity.")

src/reflect/scala/reflect/internal/TreeGen.scala

@@ -768,7 +768,7 @@ abstract class TreeGen {
   }
 
   def mkCheckIfRefutable(pat: Tree, rhs: Tree)(implicit fresh: FreshNameCreator) =
-    if (treeInfo.isVarPatternDeep(pat)) rhs
+    if (treeInfo.isVarPatternDeep(pat) || settings.ZirrefutableGeneratorPatterns) rhs
     else {
       val cases = List(
         CaseDef(pat.duplicate, EmptyTree, Literal(Constant(true))),

src/reflect/scala/reflect/internal/settings/MutableSettings.scala

@@ -55,6 +55,8 @@ abstract class MutableSettings extends AbsSettings {
   def maxClassfileName: IntSetting
 
   def isScala211: Boolean
+
+  def ZirrefutableGeneratorPatterns: BooleanSetting
 }
 
 object MutableSettings {

src/reflect/scala/reflect/runtime/Settings.scala

@@ -51,4 +51,6 @@ private[reflect] class Settings extends MutableSettings {
   val Yrecursion        = new IntSetting(0)
   val maxClassfileName  = new IntSetting(255)
   def isScala211        = true
+
+  def ZirrefutableGeneratorPatterns = new BooleanSetting(false)
 }