Name the parameter using a default

[Cherry-picked 8ba7cd3]

Author: som-snytt

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -3671,8 +3671,8 @@ final class DefaultShadowsGiven(name: Name)(using Context) extends TypeMsg(Defau
   override protected def explain(using Context): String =
     "Usually the given in scope is intended, but you must specify it after explicit `using`."
 
-final class RecurseWithDefault(using Context) extends TypeMsg(RecurseWithDefaultID):
+final class RecurseWithDefault(name: Name)(using Context) extends TypeMsg(RecurseWithDefaultID):
   override protected def msg(using Context): String =
-    i"Recursive call used a default argument."
+    i"Recursive call used a default argument for parameter $name."
   override protected def explain(using Context): String =
     "It's more explicit to pass current or modified arguments in a recursion."

compiler/src/dotty/tools/dotc/transform/TailRec.scala

@@ -327,8 +327,11 @@ class TailRec extends MiniPhase {
 
         if isRecursiveCall then
           if ctx.settings.Whas.recurseWithDefault then
-            if tree.args.exists(_.symbol.name.is(DefaultGetterName)) then
-              report.warning(RecurseWithDefault(), tree.srcPos)
+            tree.args.find(_.symbol.name.is(DefaultGetterName)) match
+            case Some(arg) =>
+              val DefaultGetterName(_, index) = arg.symbol.name: @unchecked
+              report.warning(RecurseWithDefault(calledMethod.info.firstParamNames(index)), tree.srcPos)
+            case _ =>
 
           if (inTailPosition) {
             tailrec.println("Rewriting tail recursive call:  " + tree.span)

tests/warn/i23541.check

@@ -0,0 +1,18 @@
+-- [E220] Type Warning: tests/warn/i23541.scala:29:13 ------------------------------------------------------------------
+29 |    println(f(using s = "ab")) // warn uses default instead of given // prints "ab"
+   |            ^^^^^^^^^^^^^^^^^
+   |            Argument for implicit parameter i was supplied using a default argument.
+   |
+   | longer explanation available when compiling with `-explain`
+-- [E221] Type Warning: tests/warn/i23541.scala:5:17 -------------------------------------------------------------------
+5 |  else fun(x - 1)(using p = p + x) // warn recurse uses default (instead of given passed down the stack)
+  |       ^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |       Recursive call used a default argument for parameter q.
+  |
+  | longer explanation available when compiling with `-explain`
+-- [E221] Type Warning: tests/warn/i23541.scala:9:17 -------------------------------------------------------------------
+9 |  else gun(x - 1)(p = p + x) // warn recurse uses default (value not passed down the stack)
+  |       ^^^^^^^^^^^^^^^^^^^^^
+  |       Recursive call used a default argument for parameter q.
+  |
+  | longer explanation available when compiling with `-explain`