Fix #6089: Handle super-accessors involving ()T to => T overrides

compiler/src/dotty/tools/dotc/transform/LinkScala2Impls.scala

@@ -57,7 +57,7 @@ class LinkScala2Impls extends MiniPhase with IdentityDenotTransformer { thisPhas
       )
     }
     for (sym <- mixin.info.decls) {
-      if (needsForwarder(sym) || sym.isConstructor || sym.isGetter && sym.is(Lazy) || sym.is(Method, butNot = Deferred))
+      if (needsMixinForwarder(sym) || sym.isConstructor || sym.isGetter && sym.is(Lazy) || sym.is(Method, butNot = Deferred))
         newImpl(sym.asTerm).enteredAfter(thisPhase)
     }
     // The trait is now fully augmented so the flag isn't needed anymore.

compiler/src/dotty/tools/dotc/transform/Mixin.scala

@@ -247,7 +247,7 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
             else
               initial
           // transformFollowing call is needed to make memoize & lazy vals run
-          transformFollowing(DefDef(mkForwarder(getter.asTerm), rhs))
+          transformFollowing(DefDef(mkForwarderSym(getter.asTerm), rhs))
         }
         else if (isScala2x || was(getter, ParamAccessor | Lazy)) EmptyTree
         else initial
@@ -256,13 +256,13 @@ class Mixin extends MiniPhase with SymTransformer { thisPhase =>
 
     def setters(mixin: ClassSymbol): List[Tree] =
       for (setter <- mixin.info.decls.filter(setr => setr.isSetter && !was(setr, Deferred)))
-        yield transformFollowing(DefDef(mkForwarder(setter.asTerm), unitLiteral.withSpan(cls.span)))
+        yield transformFollowing(DefDef(mkForwarderSym(setter.asTerm), unitLiteral.withSpan(cls.span)))
 
     def mixinForwarders(mixin: ClassSymbol): List[Tree] =
-      for (meth <- mixin.info.decls.toList if needsForwarder(meth))
+      for (meth <- mixin.info.decls.toList if needsMixinForwarder(meth))
       yield {
         util.Stats.record("mixin forwarders")
-        transformFollowing(polyDefDef(mkForwarder(meth.asTerm), forwarder(meth)))
+        transformFollowing(polyDefDef(mkForwarderSym(meth.asTerm), forwarderRhsFn(meth)))
       }
 
 

compiler/src/dotty/tools/dotc/transform/MixinOps.scala

@@ -18,7 +18,7 @@ class MixinOps(cls: ClassSymbol, thisPhase: DenotTransformer)(implicit ctx: Cont
     map(n => ctx.getClassIfDefined("org.junit." + n)).
     filter(_.exists)
 
-  def mkForwarder(member: TermSymbol): TermSymbol = {
+  def mkForwarderSym(member: TermSymbol): TermSymbol = {
     val res = member.copy(
       owner = cls,
       name = member.name.stripScala2LocalSuffix,
@@ -52,7 +52,7 @@ class MixinOps(cls: ClassSymbol, thisPhase: DenotTransformer)(implicit ctx: Cont
    *   - there's a class defining a method with same signature
    *   - there are multiple traits defining method with same signature
    */
-  def needsForwarder(meth: Symbol): Boolean = {
+  def needsMixinForwarder(meth: Symbol): Boolean = {
     lazy val competingMethods = competingMethodsIterator(meth).toList
 
     def needsDisambiguation = competingMethods.exists(x=> !(x is Deferred)) // multiple implementations are available
@@ -71,9 +71,18 @@ class MixinOps(cls: ClassSymbol, thisPhase: DenotTransformer)(implicit ctx: Cont
   final val PrivateOrAccessor: FlagSet = Private | Accessor
   final val PrivateOrAccessorOrDeferred: FlagSet = Private | Accessor | Deferred
 
-  def forwarder(target: Symbol): List[Type] => List[List[Tree]] => Tree =
-    targs => vrefss =>
-      superRef(target).appliedToTypes(targs).appliedToArgss(vrefss)
+  def forwarderRhsFn(target: Symbol): List[Type] => List[List[Tree]] => Tree =
+    targs => vrefss => {
+      val tapp = superRef(target).appliedToTypes(targs)
+      vrefss match {
+        case Nil | List(Nil) =>
+          // Overriding is somewhat loose about `()T` vs `=> T`, so just pick
+          // whichever makes sense for `target`
+          tapp.ensureApplied
+        case _ =>
+          tapp.appliedToArgss(vrefss)
+      }
+    }
 
   private def competingMethodsIterator(meth: Symbol): Iterator[Symbol] = {
     cls.baseClasses.iterator

compiler/src/dotty/tools/dotc/transform/ResolveSuper.scala

@@ -37,7 +37,7 @@ class ResolveSuper extends MiniPhase with IdentityDenotTransformer { thisPhase =
                                AugmentScala2Traits.name,
                                PruneErasedDefs.name) // Erased decls make `isCurrent` work incorrectly
 
-  override def changesMembers: Boolean = true // the phase adds super accessors and method forwarders
+  override def changesMembers: Boolean = true // the phase adds super accessors
 
   override def transformTemplate(impl: Template)(implicit ctx: Context): Template = {
     val cls = impl.symbol.owner.asClass
@@ -48,7 +48,7 @@ class ResolveSuper extends MiniPhase with IdentityDenotTransformer { thisPhase =
       for (superAcc <- mixin.info.decls.filter(_.isSuperAccessor))
         yield {
           util.Stats.record("super accessors")
-          polyDefDef(mkForwarder(superAcc.asTerm), forwarder(rebindSuper(cls, superAcc)))
+          polyDefDef(mkForwarderSym(superAcc.asTerm), forwarderRhsFn(rebindSuper(cls, superAcc)))
         }
 
     val overrides = mixins.flatMap(superAccessors)
@@ -63,7 +63,7 @@ class ResolveSuper extends MiniPhase with IdentityDenotTransformer { thisPhase =
       val cls = meth.owner.asClass
       val ops = new MixinOps(cls, thisPhase)
       import ops._
-      polyDefDef(meth, forwarder(rebindSuper(cls, meth)))
+      polyDefDef(meth, forwarderRhsFn(rebindSuper(cls, meth)))
     }
     else ddef
   }
@@ -92,7 +92,7 @@ object ResolveSuper {
         // of the superaccessor's type, see i5433.scala for an example where this matters
         val otherTp = other.asSeenFrom(base.typeRef).info
         val accTp = acc.asSeenFrom(base.typeRef).info
-        if (!(otherTp <:< accTp))
+        if (!(otherTp.overrides(accTp, matchLoosely = true)))
           ctx.error(IllegalSuperAccessor(base, memberName, acc, accTp, other.symbol, otherTp), base.sourcePos)
       }
 

tests/pos/i6089.scala

@@ -0,0 +1,3 @@
+trait X { override def toString = super.toString + " X" }
+trait Y { override def toString = super.toString + " Y" }
+class Z extends X with Y