Warn if implicit default shadows given

tgodzik · tgodzik · commit 58930255e8f6 · 2025-08-12T20:13:46.000+02:00
-Wrecurse-with-default warns on self-recursion with default arg. [Cherry-picked 6dac167][modified]
diff --git a/compiler/src/dotty/tools/dotc/config/ScalaSettings.scala b/compiler/src/dotty/tools/dotc/config/ScalaSettings.scala
@@ -175,6 +175,7 @@ private sealed trait WarningSettings:
   private val WNonUnitStatement = BooleanSetting("-Wnonunit-statement", "Warn when block statements are non-Unit expressions.")
   private val WenumCommentDiscard = BooleanSetting("-Wenum-comment-discard", "Warn when a comment ambiguously assigned to multiple enum cases is discarded.")
   private val WtoStringInterpolated = BooleanSetting("-Wtostring-interpolated", "Warn a standard interpolator used toString on a reference type.")
+  private val WrecurseWithDefault = BooleanSetting("-Wrecurse-with-default", "Warn when a method calls itself with a default argument.")
   private val Wunused: Setting[List[ChoiceWithHelp[String]]] = MultiChoiceHelpSetting(
     name = "-Wunused",
     helpArg = "warning",
@@ -292,6 +293,7 @@ private sealed trait WarningSettings:
     def nonUnitStatement(using Context): Boolean = allOr(WNonUnitStatement)
     def enumCommentDiscard(using Context): Boolean = allOr(WenumCommentDiscard)
     def toStringInterpolated(using Context): Boolean = allOr(WtoStringInterpolated)
+    def recurseWithDefault(using Context): Boolean = allOr(WrecurseWithDefault)
     def checkInit(using Context): Boolean = allOr(YcheckInit)
 
 /** -X "Extended" or "Advanced" settings */
diff --git a/compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala b/compiler/src/dotty/tools/dotc/reporting/ErrorMessageID.scala
@@ -233,6 +233,8 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case ErasedNotPureID // errorNumber: 217
   case IllegalErasedDefID // errorNumber: 218
   case CannotInstantiateQuotedTypeVarID // errorNumber: 219
+  case DefaultShadowsGivenID // errorNumber: 220
+  case RecurseWithDefaultID // errorNumber: 221
 
   def errorNumber = ordinal - 1
 
diff --git a/compiler/src/dotty/tools/dotc/reporting/messages.scala b/compiler/src/dotty/tools/dotc/reporting/messages.scala
@@ -3307,3 +3307,15 @@ final class NamedPatternNotApplicable(selectorType: Type)(using Context) extends
     i"Named patterns cannot be used with $selectorType, because it is not a named tuple or case class"
 
   override protected def explain(using Context): String = ""
+
+final class DefaultShadowsGiven(name: Name)(using Context) extends TypeMsg(DefaultShadowsGivenID):
+  override protected def msg(using Context): String =
+    i"Argument for implicit parameter $name was supplied using a default argument."
+  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(name: Name)(using Context) extends TypeMsg(RecurseWithDefaultID):
+  override protected def msg(using Context): String =
+    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."
diff --git a/compiler/src/dotty/tools/dotc/transform/TailRec.scala b/compiler/src/dotty/tools/dotc/transform/TailRec.scala
@@ -4,9 +4,9 @@ package transform
 import ast.{TreeTypeMap, tpd}
 import config.Printers.tailrec
 import core.*
-import Contexts.*, Flags.*, Symbols.*, Decorators.em
+import Contexts.*, Flags.*, Symbols.*, Decorators.*
 import Constants.Constant
-import NameKinds.{TailLabelName, TailLocalName, TailTempName}
+import NameKinds.{DefaultGetterName, TailLabelName, TailLocalName, TailTempName}
 import StdNames.nme
 import reporting.*
 import transform.MegaPhase.MiniPhase
@@ -323,7 +323,11 @@ class TailRec extends MiniPhase {
           method.matches(calledMethod) &&
           enclosingClass.appliedRef.widen <:< prefix.tpe.widenDealias
 
-        if (isRecursiveCall)
+        if isRecursiveCall then
+          if ctx.settings.Whas.recurseWithDefault then
+            if tree.args.exists(_.symbol.name.is(DefaultGetterName)) then
+              report.warning(RecurseWithDefault(), tree.srcPos)
+
           if (inTailPosition) {
             tailrec.println("Rewriting tail recursive call:  " + tree.span)
             rewrote = true
diff --git a/compiler/src/dotty/tools/dotc/typer/Applications.scala b/compiler/src/dotty/tools/dotc/typer/Applications.scala
@@ -674,6 +674,11 @@ trait Applications extends Compatibility {
             def implicitArg = implicitArgTree(formal, appPos.span)
 
             if !defaultArg.isEmpty then
+              if methodType.isImplicitMethod && ctx.mode.is(Mode.ImplicitsEnabled)
+              && !inferImplicitArg(formal, appPos.span).tpe.isError
+              then
+                report.warning(DefaultShadowsGiven(methodType.paramNames(n)), appPos)
+
               defaultArg.tpe.widen match
                 case _: MethodOrPoly if testOnly => matchArgs(args1, formals1, n + 1)
                 case _ => matchArgs(args1, addTyped(treeToArg(defaultArg)), n + 1)
diff --git a/scaladoc/src/dotty/tools/scaladoc/tasty/TypesSupport.scala b/scaladoc/src/dotty/tools/scaladoc/tasty/TypesSupport.scala
@@ -4,9 +4,10 @@ package tasty
 import scala.jdk.CollectionConverters._
 
 import scala.quoted._
+import scala.annotation.*
 
-import NameNormalizer._
-import SyntheticsSupport._
+import NameNormalizer.*
+import SyntheticsSupport.*
 
 trait TypesSupport:
   self: TastyParser =>
@@ -76,6 +77,7 @@ trait TypesSupport:
       case tpe => inner(tpe)
 
   // TODO #23 add support for all types signatures that make sense
+  @nowarn("id=E219")
   private def inner(
     using Quotes,
   )(
diff --git a/tests/neg/i19414-desugared.check b/tests/neg/i19414-desugared.check
@@ -1,4 +1,4 @@
--- [E172] Type Error: tests/neg/19414-desugared.scala:22:34 ------------------------------------------------------------
+-- [E172] Type Error: tests/neg/i19414-desugared.scala:22:34 -----------------------------------------------------------
 22 |  summon[BodySerializer[JsObject]] // error: Ambiguous given instances
    |                                  ^
    |No best given instance of type BodySerializer[JsObject] was found for parameter x of method summon in object Predef.
diff --git a/tests/neg/i19414-desugared.scala b/tests/neg/i19414-desugared.scala
diff --git a/tests/neg/i19414.check b/tests/neg/i19414.check
@@ -1,4 +1,4 @@
--- [E172] Type Error: tests/neg/19414.scala:15:34 ----------------------------------------------------------------------
+-- [E172] Type Error: tests/neg/i19414.scala:15:34 ---------------------------------------------------------------------
 15 |  summon[BodySerializer[JsObject]] // error: Ambiguous given instances
    |                                  ^
    |No best given instance of type BodySerializer[JsObject] was found for parameter x of method summon in object Predef.
diff --git a/tests/neg/i19414.scala b/tests/neg/i19414.scala
@@ -9,7 +9,7 @@ class Printer
 given Writer[JsValue] = ???
 given Writer[JsObject] = ???
 
-given [B: Writer](using printer: Printer = new Printer): BodySerializer[B] = ???
+given [B: Writer] => (printer: Printer = new Printer) => BodySerializer[B] = ???
 
 def f: Unit =
   summon[BodySerializer[JsObject]] // error: Ambiguous given instances
diff --git a/tests/warn/i23541.scala b/tests/warn/i23541.scala
@@ -0,0 +1,30 @@
+//> using options -Wrecurse-with-default
+
+def fun(x: Int)(using p: Int, q: Int = 0): Int =
+  if x <= 0 then p * q
+  else fun(x - 1)(using p = p + x) // warn recurse uses default (instead of given passed down the stack)
+
+def gun(x: Int)(p: Int, q: Int = 0): Int =
+  if x <= 0 then p * q
+  else gun(x - 1)(p = p + x) // warn recurse uses default (value not passed down the stack)
+
+def nested(using x: Int, y: Int = 42): Int =
+  def f: Int = nested(using x) // nowarn only self-recursive tailrec is eligible for warning
+  f
+
+def f(using s: String, i: Int = 1): String = s * i
+def g(using s: String)(using i: Int = 1): String = s * i
+
+@main def Test =
+  println(fun(3)(using p = 0, q = 1))
+  locally:
+    given String = "ab"
+    println(f) // prints "ab"
+    println(g) // prints "ab"
+  locally:
+    println(f(using s = "ab")) // prints "ab"
+    println(g(using s = "ab")) // prints "ab"
+  locally:
+    given Int = 2
+    println(f(using s = "ab")) // warn uses default instead of given // prints "ab"
+    println(g(using s = "ab")) // prints "abab"

Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`		`--- [E172] Type Error: tests/neg/19414-desugared.scala:22:34 ------------------------------------------------------------`
	`1`	`+-- [E172] Type Error: tests/neg/i19414-desugared.scala:22:34 -----------------------------------------------------------`
`2`	`2`	`22 \| summon[BodySerializer[JsObject]] // error: Ambiguous given instances`
`3`	`3`	`\| ^`
`4`	`4`	`\|No best given instance of type BodySerializer[JsObject] was found for parameter x of method summon in object Predef.`
Original file line number	Diff line number	Diff line change
`@@ -1,4 +1,4 @@`
`1`		`--- [E172] Type Error: tests/neg/19414.scala:15:34 ----------------------------------------------------------------------`
	`1`	`+-- [E172] Type Error: tests/neg/i19414.scala:15:34 ---------------------------------------------------------------------`
`2`	`2`	`15 \| summon[BodySerializer[JsObject]] // error: Ambiguous given instances`
`3`	`3`	`\| ^`
`4`	`4`	`\|No best given instance of type BodySerializer[JsObject] was found for parameter x of method summon in object Predef.`