Backport "Warn if implicit default shadows given" to 3.3 LTS

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 +323,14 @@ class TailRec extends MiniPhase {
           method.matches(calledMethod) &&
           enclosingClass.appliedRef.widen <:< prefix.tpe.widenDealias
 
-        if (isRecursiveCall)
+        if isRecursiveCall then
+          if ctx.settings.Whas.recurseWithDefault then
+            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)
             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 =>
@@ -141,24 +142,24 @@ trait TypesSupport:
             .reduceLeftOption((acc: SSignature, elem: SSignature) => acc ++ plain(", ").l ++ elem).getOrElse(List())
           ++ plain(")").l
 
-        def parseRefinedElem(name: String, info: TypeRepr, polyTyped: SSignature = Nil): SSignature = ( info match {
+        def parseRefinedElem(name: String, info: TypeRepr, polyTyped: SSignature = Nil): SSignature =
+          val ssig = info match
           case m: MethodType => {
             val paramList = getParamList(m)
             keyword("def ").l ++ plain(name).l ++ polyTyped ++ paramList ++ plain(": ").l ++ inner(m.resType)
           }
-          case t: PolyType => {
+          case t: PolyType =>
             val paramBounds = getParamBounds(t)
-            val parsedMethod = parseRefinedElem(name, t.resType)
-            if (!paramBounds.isEmpty){
+            if !paramBounds.isEmpty then
               parseRefinedElem(name, t.resType, plain("[").l ++ paramBounds ++ plain("]").l)
-            } else parseRefinedElem(name, t.resType)
-          }
+            else parseRefinedElem(name, t.resType, polyTyped = Nil)
           case ByNameType(tp) => keyword("def ").l ++ plain(s"$name: ").l ++ inner(tp)
           case t: TypeBounds => keyword("type ").l ++ plain(name).l ++ inner(t)
           case t: TypeRef => keyword("val ").l ++ plain(s"$name: ").l ++ inner(t)
           case t: TermRef => keyword("val ").l ++ plain(s"$name: ").l ++ inner(t)
           case other => noSupported(s"Not supported type in refinement $info")
-        } ) ++ plain("; ").l
+
+          ssig ++ plain("; ").l
 
         def parsePolyFunction(info: TypeRepr): SSignature = info match {
           case t: PolyType =>
@@ -225,6 +226,7 @@ trait TypesSupport:
         }) ++ plain("]").l
 
       case tp @ TypeRef(qual, typeName) =>
+        inline def wrapping = shouldWrapInParens(inner = qual, outer = tp, isLeft = true)
         qual match {
           case r: RecursiveThis => tpe(s"this.$typeName").l
           case t if skipPrefix(t, elideThis) =>
@@ -245,17 +247,17 @@ trait TypesSupport:
                   case _ => tpe(tp.typeSymbol)
               case Some(_) => tpe(tp.typeSymbol)
               case None =>
-                val sig = inParens(inner(qual)(using skipTypeSuffix = true), shouldWrapInParens(qual, tp, true))
+                val sig = inParens(inner(qual)(using indent = indent, skipTypeSuffix = true), wrapping)
                 sig ++ plain(".").l ++ tpe(tp.typeSymbol)
           case _ =>
-            val sig = inParens(inner(qual), shouldWrapInParens(qual, tp, true))
+            val sig = inParens(inner(qual, skipThisTypePrefix), wrapping)
             sig ++ keyword("#").l ++ tpe(tp.typeSymbol)
         }
 
       case tr @ TermRef(qual, typeName) =>
         val prefix = qual match
           case t if skipPrefix(t, elideThis) => Nil
-          case tp => inner(tp)(using skipTypeSuffix = true) ++ plain(".").l
+          case tp => inner(tp)(using indent = indent, skipTypeSuffix = true) ++ plain(".").l
         val suffix = if skipTypeSuffix then Nil else List(plain("."), keyword("type"))
         val typeSig = tr.termSymbol.tree match
           case vd: ValDef if tr.termSymbol.flags.is(Flags.Module) =>
@@ -274,9 +276,9 @@ trait TypesSupport:
         val spaces = " " * (indent)
         val casesTexts = cases.flatMap {
           case MatchCase(from, to) =>
-            keyword(caseSpaces + "case ").l ++ inner(from) ++ keyword(" => ").l ++ inner(to)(using indent = indent + 2) ++ plain("\n").l
+            keyword(caseSpaces + "case ").l ++ inner(from) ++ keyword(" => ").l ++ inner(to)(using indent = indent + 2, skipTypeSuffix = skipTypeSuffix) ++ plain("\n").l
           case TypeLambda(_, _, MatchCase(from, to)) =>
-            keyword(caseSpaces + "case ").l ++ inner(from) ++ keyword(" => ").l ++ inner(to)(using indent = indent + 2) ++ plain("\n").l
+            keyword(caseSpaces + "case ").l ++ inner(from) ++ keyword(" => ").l ++ inner(to)(using indent = indent + 2, skipTypeSuffix = skipTypeSuffix) ++ plain("\n").l
         }
         inner(sc) ++ keyword(" match ").l ++ plain("{\n").l ++ casesTexts ++ plain(spaces + "}").l
 

tests/neg/19414-desugared.check → 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.check → 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.

tests/neg/19414.scala → 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

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`

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"