Warn if implicit default shadows given

-Wrecurse-with-default warns on self-recursion with default arg.

[Cherry-picked 6dac167][modified]

Author: tgodzik

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 */

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
 

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."

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

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)

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,
   )(

tests/neg/19414-desugared.check renamed to 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.

tests/neg/19414-desugared.scala renamed to tests/neg/i19414-desugared.scala

@@ -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.

tests/neg/19414.check renamed to tests/neg/i19414.check

@@ -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