Warn if implicit default shadows given

Author: som-snytt

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

@@ -232,6 +232,7 @@ enum ErrorMessageID(val isActive: Boolean = true) extends java.lang.Enum[ErrorMe
   case UnnecessaryNN // errorNumber: 216
   case ErasedNotPureID // errorNumber: 217
   case IllegalErasedDefID // errorNumber: 218
+  case DefaultShadowsGivenID // errorNumber: 219
 
   def errorNumber = ordinal - 1
 

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

@@ -3652,3 +3652,9 @@ final class IllegalErasedDef(sym: Symbol)(using Context) extends TypeMsg(Illegal
   override protected def explain(using Context): String =
     "Only non-lazy immutable values can be `erased`"
 end IllegalErasedDef
+
+final class DefaultShadowsGiven(name: Name)(using Context) extends TypeMsg(DefaultShadowsGivenID):
+  override protected def msg(using Context): String =
+    i"Argument for parameter $name was supplied using a default argument."
+  override protected def explain(using Context): String =
+    "Usually it's intended to prefer the given in scope, but you must specify it explicitly."

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -774,6 +774,11 @@ trait Applications extends Compatibility {
               methodType.isImplicitMethod && (applyKind == ApplyKind.Using || failEmptyArgs)
 
             if !defaultArg.isEmpty then
+              if sym == ctx.owner
+              || methodType.isImplicitMethod && ctx.mode.is(Mode.ImplicitsEnabled)
+                 && inferImplicitArg(formal, appPos.span).tpe.isError == false
+              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)

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

@@ -0,0 +1,25 @@
+
+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 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"