Fix #8802: Use wildcards for result type approximation of implicits

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

@@ -534,22 +534,23 @@ object ProtoTypes {
     newTypeVar(TypeBounds.upper(AndType(tp.widenExpr, defn.SingletonClass.typeRef)))
 
   /** The result type of `mt`, where all references to parameters of `mt` are
-   *  replaced by either wildcards (if typevarsMissContext) or TypeParamRefs.
+   *  replaced by either wildcards or TypeParamRefs.
    */
-  def resultTypeApprox(mt: MethodType)(using Context): Type =
-    if (mt.isResultDependent) {
+  def resultTypeApprox(mt: MethodType, wildcardOnly: Boolean = false)(using Context): Type =
+    if mt.isResultDependent then
       def replacement(tp: Type) =
-        if (ctx.mode.is(Mode.TypevarsMissContext) ||
-            !tp.widenExpr.isValueTypeOrWildcard) WildcardType
+        if wildcardOnly
+           || ctx.mode.is(Mode.TypevarsMissContext)
+           || !tp.widenExpr.isValueTypeOrWildcard
+        then WildcardType
         else newDepTypeVar(tp)
       mt.resultType.substParams(mt, mt.paramInfos.map(replacement))
-    }
     else mt.resultType
 
   /** The normalized form of a type
    *   - unwraps polymorphic types, tracking their parameters in the current constraint
    *   - skips implicit parameters of methods and functions;
-   *     if result type depends on implicit parameter, replace with fresh type dependent parameter.
+   *     if result type depends on implicit parameter, replace with wildcard.
    *   - converts non-dependent method types to the corresponding function types
    *     unless the expected type is an ApplyingProto or IgnoredProto.
    *   - dereferences parameterless method types
@@ -568,7 +569,7 @@ object ProtoTypes {
       case poly: PolyType =>
         normalize(constrained(poly).resultType, pt)
       case mt: MethodType =>
-        if (mt.isImplicitMethod) normalize(resultTypeApprox(mt), pt)
+        if (mt.isImplicitMethod) normalize(resultTypeApprox(mt, wildcardOnly = true), pt)
         else if (mt.isResultDependent) tp
         else {
           val rt = normalize(mt.resultType, pt)

tests/pos/i8802.scala

@@ -0,0 +1,17 @@
+trait Foo[A, B] {
+  type Out
+}
+
+object Test {
+
+  type Bar[A]
+
+  def unit: Bar[Unit] = ???
+  def product[A, B](fst: Bar[A], snd: Bar[B])(implicit foo: Foo[A, B]): Bar[foo.Out] = ???
+
+  implicit def foo[A]: Foo[A, Unit] { type Out = A } = ???
+
+  def check[A](bar: Bar[A])(a: A): Unit = {}
+
+  check(product(unit, unit))(()) // error
+}

tests/pos/i8825.scala

@@ -0,0 +1,24 @@
+sealed trait Nat
+case class Succ[N <: Nat](n: N) extends Nat
+case object Zero extends Nat
+type Zero = Zero.type
+type One = Succ[Zero]
+
+sealed trait HList
+case class HCons[+H, +T <: HList](head: H, tail: T) extends HList
+case object HNil extends HList
+type HNil = HNil.type
+
+trait Length[L <: HList] {
+  type Out <: Nat
+}
+object Length {
+  type Aux[L <: HList, Out0 <: Nat] = Length[L] { type Out = Out0 }
+  def instance[L <: HList, Out0 <: Nat]: Aux[L, Out0] = new Length[L] { type Out = Out0 }
+
+  given hnilLength as Aux[HNil, Zero] = instance
+  given hconsLength[H, T <: HList] (using length: Length[T]) as Aux[HCons[H, T], Succ[length.Out]] = instance // (*)
+  //given hconsLength[H, T <: HList, N <: Nat] (using length: Aux[T, N]) as Aux[HCons[H, T], Succ[N]] = instance // (**)
+}
+
+val test = summon[Length.Aux[HCons[Int, HNil], One]]