3 files changed, 25 insertions, 5 deletions
diff --git a/src/reflect/scala/reflect/internal/StdAttachments.scala b/src/reflect/scala/reflect/internal/StdAttachments.scala
index fc49de1cf6..f72c1eb1b3 100644
--- a/src/reflect/scala/reflect/internal/StdAttachments.scala
+++ b/src/reflect/scala/reflect/internal/StdAttachments.scala
@@ -57,6 +57,12 @@ trait StdAttachments {
    */
   case object BackquotedIdentifierAttachment extends PlainAttachment
 
+  /** Indicates that the host `Ident` has been created from a pattern2 binding, `case x @ p`.
+   *  In the absence of named parameters in patterns, allows nuanced warnings for unused variables.
+   *  Hence, `case X(x = _) =>` would not warn; for now, `case X(x @ _) =>` is documentary if x is unused.
+   */
+  case object AtBoundIdentifierAttachment extends PlainAttachment
+
   /** Identifies trees are either result or intermediate value of for loop desugaring.
    */
   case object ForAttachment extends PlainAttachment
diff --git a/src/reflect/scala/reflect/internal/TreeGen.scala b/src/reflect/scala/reflect/internal/TreeGen.scala
index 4fecaf70df..ade9ee84ac 100644
--- a/src/reflect/scala/reflect/internal/TreeGen.scala
+++ b/src/reflect/scala/reflect/internal/TreeGen.scala
@@ -797,7 +797,7 @@ abstract class TreeGen {
 
   /** Create tree for for-comprehension generator <val pat0 <- rhs0> */
   def mkGenerator(pos: Position, pat: Tree, valeq: Boolean, rhs: Tree)(implicit fresh: FreshNameCreator): Tree = {
-    val pat1 = patvarTransformer.transform(pat)
+    val pat1 = patvarTransformerForFor.transform(pat)
     if (valeq) ValEq(pat1, rhs).setPos(pos)
     else ValFrom(pat1, mkCheckIfRefutable(pat1, rhs)).setPos(pos)
   }
@@ -894,11 +894,15 @@ abstract class TreeGen {
    *    x                  becomes      x @ _
    *    x: T               becomes      x @ (_: T)
    */
-  object patvarTransformer extends Transformer {
+  class PatvarTransformer(forFor: Boolean) extends Transformer {
     override def transform(tree: Tree): Tree = tree match {
-      case Ident(name) if (treeInfo.isVarPattern(tree) && name != nme.WILDCARD) =>
-        atPos(tree.pos)(Bind(name, atPos(tree.pos.focus) (Ident(nme.WILDCARD))))
-      case Typed(id @ Ident(name), tpt) if (treeInfo.isVarPattern(id) && name != nme.WILDCARD) =>
+      case Ident(name) if treeInfo.isVarPattern(tree) && name != nme.WILDCARD =>
+        atPos(tree.pos) {
+          val b = Bind(name, atPos(tree.pos.focus) (Ident(nme.WILDCARD)))
+          if (!forFor && isPatVarWarnable) b
+          else b updateAttachment AtBoundIdentifierAttachment
+        }
+      case Typed(id @ Ident(name), tpt) if treeInfo.isVarPattern(id) && name != nme.WILDCARD =>
         atPos(tree.pos.withPoint(id.pos.point)) {
           Bind(name, atPos(tree.pos.withStart(tree.pos.point)) {
             Typed(Ident(nme.WILDCARD), tpt)
@@ -919,6 +923,15 @@ abstract class TreeGen {
     }
   }
 
+  /** Can be overridden to depend on settings.warnUnusedPatvars. */
+  def isPatVarWarnable: Boolean = true
+
+  /** Not in for comprehensions, whether to warn unused pat vars depends on flag. */
+  object patvarTransformer       extends PatvarTransformer(forFor = false)
+
+  /** Tag pat vars in for comprehensions. */
+  object patvarTransformerForFor extends PatvarTransformer(forFor = true)
+
   // annotate the expression with @unchecked
   def mkUnchecked(expr: Tree): Tree = atPos(expr.pos) {
     // This can't be "Annotated(New(UncheckedClass), expr)" because annotations
diff --git a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
index 9138ed3f02..b455a08036 100644
--- a/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
+++ b/src/reflect/scala/reflect/runtime/JavaUniverseForce.scala
@@ -40,6 +40,7 @@ trait JavaUniverseForce { self: runtime.JavaUniverse  =>
     this.SAMFunction
     this.DelambdafyTarget
     this.BackquotedIdentifierAttachment
+    this.AtBoundIdentifierAttachment
     this.ForAttachment
     this.SyntheticUnitAttachment
     this.SubpatternsAttachment