Merge pull request #5816 from adriaanm/userdefined-apply-212

Allow user-defined `[un]apply` in case companion

10 files changed, 451 insertions, 213 deletions

diff --git a/spec/05-classes-and-objects.md b/spec/05-classes-and-objects.md
index 5bd520589d..ffb65979f7 100644
--- a/spec/05-classes-and-objects.md
+++ b/spec/05-classes-and-objects.md
@@ -854,9 +854,8 @@ a `val` or `var` modifier. Hence, an accessor
 definition for the parameter is [generated](#class-definitions).
 
 A case class definition of `$c$[$\mathit{tps}\,$]($\mathit{ps}_1\,$)$\ldots$($\mathit{ps}_n$)` with type
-parameters $\mathit{tps}$ and value parameters $\mathit{ps}$ implicitly
-generates an [extractor object](08-pattern-matching.html#extractor-patterns) which is
-defined as follows:
+parameters $\mathit{tps}$ and value parameters $\mathit{ps}$ implies
+the definition of a companion object, which serves as an [extractor object](08-pattern-matching.html#extractor-patterns). It has the following shape:
 
 ```scala
 object $c$ {
@@ -873,11 +872,13 @@ each $\mathit{xs}\_i$ denotes the parameter names of the parameter
 section $\mathit{ps}\_i$, and
 $\mathit{xs}\_{11}, \ldots , \mathit{xs}\_{1k}$ denote the names of all parameters
 in the first parameter section $\mathit{xs}\_1$.
-If a type parameter section is missing in the
-class, it is also missing in the `apply` and
-`unapply` methods.
-The definition of `apply` is omitted if class $c$ is
-`abstract`.
+If a type parameter section is missing in the class, it is also missing in the `apply` and `unapply` methods.
+
+If the companion object $c$ is already defined,
+the  `apply` and `unapply` methods are added to the existing object.
+If the object $c$ already has a [matching](#definition-matching)
+`apply` (or `unapply`) member, no new definition is added.
+The definition of `apply` is omitted if class $c$ is `abstract`.
 
 If the case class definition contains an empty value parameter list, the
 `unapply` method returns a `Boolean` instead of an `Option` type and
@@ -890,9 +891,6 @@ def unapply[$\mathit{tps}\,$]($x$: $c$[$\mathit{tps}\,$]) = x ne null
 The name of the `unapply` method is changed to `unapplySeq` if the first
 parameter section $\mathit{ps}_1$ of $c$ ends in a
 [repeated parameter](04-basic-declarations-and-definitions.html#repeated-parameters).
-If a companion object $c$ exists already, no new object is created,
-but the `apply` and `unapply` methods are added to the existing
-object instead.
 
 A method named `copy` is implicitly added to every case class unless the
 class already has a member (directly defined or inherited) with that name, or the
diff --git a/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala b/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
index 0f257d3717..72d186b301 100644
--- a/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/MethodSynthesis.scala
@@ -146,8 +146,8 @@ trait MethodSynthesis {
         // if there's no field symbol, the ValDef tree receives the getter symbol and thus is not a synthetic
         if (fieldSym != NoSymbol) {
           context.unit.synthetics(getterSym) = getter.derivedTree(getterSym)
-          getterSym setInfo namer.accessorTypeCompleter(tree, tree.tpt.isEmpty, isBean = false, isSetter = false)
-        } else getterSym setInfo namer.valTypeCompleter(tree)
+          getterSym setInfo new namer.AccessorTypeCompleter(tree, tree.tpt.isEmpty, isBean = false, isSetter = false)
+        } else getterSym setInfo new namer.ValTypeCompleter(tree)
 
         enterInScope(getterSym)
 
@@ -155,17 +155,17 @@ trait MethodSynthesis {
           val setter = Setter(tree)
           val setterSym = setter.createSym
           context.unit.synthetics(setterSym) = setter.derivedTree(setterSym)
-          setterSym setInfo namer.accessorTypeCompleter(tree, tree.tpt.isEmpty, isBean = false, isSetter = true)
+          setterSym setInfo new namer.AccessorTypeCompleter(tree, tree.tpt.isEmpty, isBean = false, isSetter = true)
           enterInScope(setterSym)
         }
 
         // TODO: delay emitting the field to the fields phase (except for private[this] vals, which only get a field and no accessors)
         if (fieldSym != NoSymbol) {
-          fieldSym setInfo namer.valTypeCompleter(tree)
+          fieldSym setInfo new namer.ValTypeCompleter(tree)
           enterInScope(fieldSym)
         }
       } else {
-        getterSym setInfo namer.valTypeCompleter(tree)
+        getterSym setInfo new namer.ValTypeCompleter(tree)
         enterInScope(getterSym)
       }
 
@@ -208,11 +208,11 @@ trait MethodSynthesis {
           sym
         }
 
-        val getterCompleter = namer.accessorTypeCompleter(tree, missingTpt, isBean = true, isSetter = false)
+        val getterCompleter = new namer.AccessorTypeCompleter(tree, missingTpt, isBean = true, isSetter = false)
         enterInScope(deriveBeanAccessor(if (hasBeanProperty) "get" else "is") setInfo getterCompleter)
 
         if (tree.mods.isMutable) {
-          val setterCompleter = namer.accessorTypeCompleter(tree, missingTpt, isBean = true, isSetter = true)
+          val setterCompleter = new namer.AccessorTypeCompleter(tree, missingTpt, isBean = true, isSetter = true)
           enterInScope(deriveBeanAccessor("set") setInfo setterCompleter)
         }
       }
@@ -221,9 +221,9 @@ trait MethodSynthesis {
 
     def enterImplicitWrapper(classDef: ClassDef): Unit = {
       val methDef = factoryMeth(classDef.mods & AccessFlags | METHOD | IMPLICIT | SYNTHETIC, classDef.name.toTermName, classDef)
-      val methSym = assignAndEnterSymbol(methDef)
+      val methSym = enterInScope(assignMemberSymbol(methDef))
       context.unit.synthetics(methSym) = methDef
-      methSym setInfo implicitFactoryMethodCompleter(methDef, classDef.symbol, completerOf(methDef).asInstanceOf[LockingTypeCompleter])
+      methSym setInfo implicitFactoryMethodCompleter(methDef, classDef.symbol)
     }
 
 
diff --git a/src/compiler/scala/tools/nsc/typechecker/Namers.scala b/src/compiler/scala/tools/nsc/typechecker/Namers.scala
index 095cc555e9..fee56cfc13 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Namers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Namers.scala
@@ -103,14 +103,10 @@ trait Namers extends MethodSynthesis {
       else newNamer(cx)
     }
 
-    def enterValueParams(vparamss: List[List[ValDef]]): List[List[Symbol]] = {
+    def enterValueParams(vparamss: List[List[ValDef]]): List[List[Symbol]] =
       mmap(vparamss) { param =>
-        val sym = assignSymbol(param, param.name, mask = ValueParameterFlags)
-        setPrivateWithin(param, sym)
-        enterInScope(sym)
-        sym setInfo monoTypeCompleter(param)
+        enterInScope(assignMemberSymbol(param, mask = ValueParameterFlags)) setInfo new MonoTypeCompleter(param)
       }
-    }
 
     protected def owner       = context.owner
     def contextFile = context.unit.source.file
@@ -286,9 +282,7 @@ trait Namers extends MethodSynthesis {
           case tree @ DefDef(_, _, _, _, _, _)               => enterDefDef(tree)
           case tree @ TypeDef(_, _, _, _)                    => enterTypeDef(tree)
           case DocDef(_, defn)                               => enterSym(defn)
-          case tree @ Import(_, _)                           =>
-            assignSymbol(tree)
-            returnContext = context.make(tree)
+          case tree @ Import(_, _)                           => enterImport(tree); returnContext = context.make(tree)
           case _ =>
         }
         returnContext
@@ -299,25 +293,15 @@ trait Namers extends MethodSynthesis {
       }
     }
 
-    /** Creates a new symbol and assigns it to the tree, returning the symbol
-     */
-    def assignSymbol(tree: Tree): Symbol =
-      logAssignSymbol(tree, tree match {
-        case PackageDef(pid, _) => createPackageSymbol(tree.pos, pid)
-        case imp: Import        => createImportSymbol(imp)
-        case mdef: MemberDef    => createMemberSymbol(mdef, mdef.name, -1L)
-        case _                  => abort("Unexpected tree: " + tree)
-      })
-    def assignSymbol(tree: MemberDef, name: Name, mask: Long): Symbol =
-      logAssignSymbol(tree, createMemberSymbol(tree, name, mask))
-
-    def assignAndEnterSymbol(tree: MemberDef): Symbol = {
-      val sym = assignSymbol(tree, tree.name, -1L)
+    def assignMemberSymbol(tree: MemberDef, mask: Long = -1L): Symbol = {
+      val sym = createMemberSymbol(tree, tree.name, mask)
       setPrivateWithin(tree, sym)
-      enterInScope(sym)
+      tree.symbol = sym
+      sym
     }
+
     def assignAndEnterFinishedSymbol(tree: MemberDef): Symbol = {
-      val sym = assignAndEnterSymbol(tree)
+      val sym = enterInScope(assignMemberSymbol(tree))
       sym setInfo completerOf(tree)
       // log("[+info] " + sym.fullLocationString)
       sym
@@ -329,19 +313,6 @@ trait Namers extends MethodSynthesis {
       sym
     }
 
-    private def logAssignSymbol(tree: Tree, sym: Symbol): Symbol = {
-      if (isPastTyper) sym.name.toTermName match {
-        case nme.IMPORT | nme.OUTER | nme.ANON_CLASS_NAME | nme.ANON_FUN_NAME | nme.CONSTRUCTOR => ()
-        case _                                                                                  =>
-          tree match {
-            case md: DefDef => log("[+symbol] " + sym.debugLocationString)
-            case _          =>
-          }
-      }
-      tree.symbol = sym
-      sym
-    }
-
     /** Create a new symbol at the context owner based on the given tree.
      *  A different name can be given.  If the modifier flags should not be
      *  be transferred to the symbol as they are, supply a mask containing
@@ -366,8 +337,10 @@ trait Namers extends MethodSynthesis {
       }
     }
 
-    def createImportSymbol(tree: Import) =
-      NoSymbol.newImport(tree.pos) setInfo (namerOf(tree.symbol) importTypeCompleter tree)
+    def createImportSymbol(tree: Import) = {
+      val importNamer = namerOf(tree.symbol)
+      NoSymbol.newImport(tree.pos) setInfo new importNamer.ImportTypeCompleter(tree)
+    }
 
     /** All PackageClassInfoTypes come from here. */
     def createPackageSymbol(pos: Position, pid: RefTree): Symbol = {
@@ -419,7 +392,7 @@ trait Namers extends MethodSynthesis {
           clearRenamedCaseAccessors(existing)
           existing
         }
-        else assignAndEnterSymbol(tree) setFlag inConstructorFlag
+        else enterInScope(assignMemberSymbol(tree)) setFlag inConstructorFlag
       }
       clazz match {
         case csym: ClassSymbol if csym.isTopLevel => enterClassSymbol(tree, csym)
@@ -457,7 +430,8 @@ trait Namers extends MethodSynthesis {
 
     def enterModuleDef(tree: ModuleDef) = {
       val sym = enterModuleSymbol(tree)
-      sym.moduleClass setInfo namerOf(sym).moduleClassTypeCompleter(tree)
+      val mcsNamer = namerOf(sym)
+      sym.moduleClass setInfo new mcsNamer.ModuleClassTypeCompleter(tree)
       sym setInfo completerOf(tree)
       validateCompanionDefs(tree)
       sym
@@ -466,9 +440,10 @@ trait Namers extends MethodSynthesis {
     /** Enter a module symbol.
      */
     def enterModuleSymbol(tree : ModuleDef): Symbol = {
-      var m: Symbol = context.scope lookupModule tree.name
       val moduleFlags = tree.mods.flags | MODULE
-      if (m.isModule && !m.hasPackageFlag && inCurrentScope(m) && (currentRun.canRedefine(m) || m.isSynthetic)) {
+
+      val existingModule = context.scope lookupModule tree.name
+      if (existingModule.isModule && !existingModule.hasPackageFlag && inCurrentScope(existingModule) && (currentRun.canRedefine(existingModule) || existingModule.isSynthetic)) {
         // This code accounts for the way the package objects found in the classpath are opened up
         // early by the completer of the package itself. If the `packageobjects` phase then finds
         // the same package object in sources, we have to clean the slate and remove package object
@@ -476,21 +451,24 @@ trait Namers extends MethodSynthesis {
         //
         // TODO SI-4695 Pursue the approach in https://github.com/scala/scala/pull/2789 that avoids
         //      opening up the package object on the classpath at all if one exists in source.
-        if (m.isPackageObject) {
-          val packageScope = m.enclosingPackageClass.rawInfo.decls
-          packageScope.foreach(mem => if (mem.owner != m.enclosingPackageClass) packageScope unlink mem)
+        if (existingModule.isPackageObject) {
+          val packageScope = existingModule.enclosingPackageClass.rawInfo.decls
+          packageScope.foreach(mem => if (mem.owner != existingModule.enclosingPackageClass) packageScope unlink mem)
         }
-        updatePosFlags(m, tree.pos, moduleFlags)
-        setPrivateWithin(tree, m)
-        m.moduleClass andAlso (setPrivateWithin(tree, _))
-        context.unit.synthetics -= m
-        tree.symbol = m
+        updatePosFlags(existingModule, tree.pos, moduleFlags)
+        setPrivateWithin(tree, existingModule)
+        existingModule.moduleClass andAlso (setPrivateWithin(tree, _))
+        context.unit.synthetics -= existingModule
+        tree.symbol = existingModule
       }
       else {
-        m = assignAndEnterSymbol(tree)
+        enterInScope(assignMemberSymbol(tree))
+        val m = tree.symbol
         m.moduleClass setFlag moduleClassFlags(moduleFlags)
         setPrivateWithin(tree, m.moduleClass)
       }
+
+      val m = tree.symbol
       if (m.isTopLevel && !m.hasPackageFlag) {
         m.moduleClass.associatedFile = contextFile
         currentRun.symSource(m) = m.moduleClass.sourceFile
@@ -613,13 +591,11 @@ trait Namers extends MethodSynthesis {
       noDuplicates(selectors map (_.rename), AppearsTwice)
     }
 
-    def enterCopyMethod(copyDef: DefDef): Symbol = {
-      val sym      = copyDef.symbol
-      val lazyType = completerOf(copyDef)
-
+    def copyMethodCompleter(copyDef: DefDef): TypeCompleter = {
       /* Assign the types of the class parameters to the parameters of the
-       * copy method. See comment in `Unapplies.caseClassCopyMeth` */
-      def assignParamTypes() {
+       * copy method. See comment in `Unapplies.caseClassCopyMeth`
+       */
+      def assignParamTypes(copyDef: DefDef, sym: Symbol) {
         val clazz = sym.owner
         val constructorType = clazz.primaryConstructor.tpe
         val subst = new SubstSymMap(clazz.typeParams, copyDef.tparams map (_.symbol))
@@ -632,16 +608,84 @@ trait Namers extends MethodSynthesis {
         )
       }
 
-      sym setInfo {
-        mkTypeCompleter(copyDef) { sym =>
-          assignParamTypes()
-          lazyType complete sym
+      new CompleterWrapper(completerOf(copyDef)) {
+        override def complete(sym: Symbol): Unit = {
+          assignParamTypes(tree.asInstanceOf[DefDef], sym)
+          super.complete(sym)
         }
       }
     }
 
+    // for apply/unapply, which may need to disappear when they clash with a user-defined method of matching signature
+    def applyUnapplyMethodCompleter(un_applyDef: DefDef, companionContext: Context): TypeCompleter =
+      new CompleterWrapper(completerOf(un_applyDef)) {
+        override def complete(sym: Symbol): Unit = {
+          assert(sym hasAllFlags CASE | SYNTHETIC, sym.defString)
+
+          super.complete(sym)
+
+          // don't propagate e.g. @volatile annot to apply's argument
+          def retainOnlyParamAnnots(param: Symbol) =
+            param setAnnotations (param.annotations filter AnnotationInfo.mkFilter(ParamTargetClass, defaultRetention = false))
+
+          sym.info.paramss.foreach(_.foreach(retainOnlyParamAnnots))
+
+          // owner won't be locked
+          val ownerInfo = companionContext.owner.info
+
+          // If there's a same-named locked symbol, we're currently completing its signature.
+          // If `scopePartiallyCompleted`, the program is known to have a type error, since
+          // this means a user-defined method is missing a result type while its rhs refers to `sym` or an overload.
+          // This is an error because overloaded/recursive methods must have a result type.
+          // The method would be overloaded if its signature, once completed, would not match the synthetic method's,
+          // or recursive if it turned out we should unlink our synthetic method (matching sig).
+          // In any case, error out. We don't unlink the symbol so that `symWasOverloaded` says yes,
+          // which would be wrong if the method is in fact recursive, but it seems less confusing.
+          val scopePartiallyCompleted = new HasMember(ownerInfo, sym.name, BridgeFlags | SYNTHETIC, LOCKED).apply()
+
+          // Check `scopePartiallyCompleted` first to rule out locked symbols from the owner.info.member call,
+          // as FindMember will call info on a locked symbol (while checking type matching to assemble an overloaded type),
+          // and throw a TypeError, so that we are aborted.
+          // Do not consider deferred symbols, as suppressing our concrete implementation would be an error regardless
+          // of whether the signature matches (if it matches, we omitted a valid implementation, if it doesn't,
+          // we would get an error for the missing implementation it isn't implemented by some overload other than our synthetic one)
+          val suppress = scopePartiallyCompleted || {
+            // can't exclude deferred members using DEFERRED flag here (TODO: why?)
+            val userDefined = ownerInfo.memberBasedOnName(sym.name, BridgeFlags | SYNTHETIC)
+
+            (userDefined != NoSymbol) && {
+              assert(userDefined != sym)
+              val alts = userDefined.alternatives // could be just the one, if this member isn't overloaded
+              // don't compute any further `memberInfo`s if there's an error somewhere
+              alts.exists(_.isErroneous) || {
+                val self = companionContext.owner.thisType
+                val memberInfo = self.memberInfo(sym)
+                alts.exists(alt => !alt.isDeferred && (self.memberInfo(alt) matches memberInfo))
+              }
+            }
+          }
+
+          if (suppress) {
+            sym setInfo ErrorType
+            sym setFlag IS_ERROR
+
+            // Don't unlink in an error situation to generate less confusing error messages.
+            // Ideally, our error reporting would distinguish overloaded from recursive user-defined apply methods without signature,
+            // but this would require some form of partial-completion of method signatures, so that we can
+            // know what the argument types were, even though we can't complete the result type, because
+            // we hit a cycle while trying to compute it (when we get here with locked user-defined symbols, we
+            // are in the complete for that symbol, and thus the locked symbol has not yet received enough info;
+            // I hesitate to provide more info, because it would involve a WildCard or something for its result type,
+            // which could upset other code paths)
+            if (!scopePartiallyCompleted)
+              companionContext.scope.unlink(sym)
+          }
+        }
+      }
+
     def completerOf(tree: MemberDef): TypeCompleter = {
-      val mono = namerOf(tree.symbol) monoTypeCompleter tree
+      val treeNamer = namerOf(tree.symbol)
+      val mono = new treeNamer.MonoTypeCompleter(tree)
       val tparams = treeInfo.typeParameters(tree)
       if (tparams.isEmpty) mono
       else {
@@ -675,25 +719,34 @@ trait Namers extends MethodSynthesis {
     }
 
     def enterPackage(tree: PackageDef) {
-      val sym = assignSymbol(tree)
+      val sym = createPackageSymbol(tree.pos, tree.pid)
+      tree.symbol = sym
       newNamer(context.make(tree, sym.moduleClass, sym.info.decls)) enterSyms tree.stats
     }
+
+    private def enterImport(tree: Import) = {
+      val sym = createImportSymbol(tree)
+      tree.symbol = sym
+    }
+
     def enterTypeDef(tree: TypeDef) = assignAndEnterFinishedSymbol(tree)
 
     def enterDefDef(tree: DefDef): Unit = tree match {
       case DefDef(_, nme.CONSTRUCTOR, _, _, _, _) =>
         assignAndEnterFinishedSymbol(tree)
-      case DefDef(mods, name, tparams, _, _, _) =>
+      case DefDef(mods, name, _, _, _, _) =>
         val bridgeFlag = if (mods hasAnnotationNamed tpnme.bridgeAnnot) BRIDGE | ARTIFACT else 0
-        val sym = assignAndEnterSymbol(tree) setFlag bridgeFlag
+        val sym = enterInScope(assignMemberSymbol(tree)) setFlag bridgeFlag
 
-        if (name == nme.copy && sym.isSynthetic)
-          enterCopyMethod(tree)
-        else if (name == nme.apply && sym.hasAllFlags(SYNTHETIC | CASE))
-          sym setInfo caseApplyMethodCompleter(tree, completerOf(tree).asInstanceOf[LockingTypeCompleter])
-        else
-          sym setInfo completerOf(tree)
-    }
+        val completer =
+          if (sym hasFlag SYNTHETIC) {
+            if (name == nme.copy) copyMethodCompleter(tree)
+            else if (sym hasFlag CASE) applyUnapplyMethodCompleter(tree, context)
+            else completerOf(tree)
+          } else completerOf(tree)
+
+        sym setInfo completer
+      }
 
     def enterClassDef(tree: ClassDef) {
       val ClassDef(mods, _, _, impl) = tree
@@ -762,125 +815,129 @@ trait Namers extends MethodSynthesis {
       NoSymbol
     }
 
-    def monoTypeCompleter(tree: MemberDef) = mkTypeCompleter(tree) { sym =>
-      // this early test is there to avoid infinite baseTypes when
-      // adding setters and getters --> bug798
-      // It is a def in an attempt to provide some insulation against
-      // uninitialized symbols misleading us. It is not a certainty
-      // this accomplishes anything, but performance is a non-consideration
-      // on these flag checks so it can't hurt.
-      def needsCycleCheck = sym.isNonClassType && !sym.isParameter && !sym.isExistential
-
-      val annotations = annotSig(tree.mods.annotations)
+    def monoTypeCompleter(tree: MemberDef) = new MonoTypeCompleter(tree)
+    class MonoTypeCompleter(tree: MemberDef) extends TypeCompleterBase(tree) {
+      override def completeImpl(sym: Symbol): Unit = {
+        // this early test is there to avoid infinite baseTypes when
+        // adding setters and getters --> bug798
+        // It is a def in an attempt to provide some insulation against
+        // uninitialized symbols misleading us. It is not a certainty
+        // this accomplishes anything, but performance is a non-consideration
+        // on these flag checks so it can't hurt.
+        def needsCycleCheck = sym.isNonClassType && !sym.isParameter && !sym.isExistential
+
+        val annotations = annotSig(tree.mods.annotations)
+
+        val tp = typeSig(tree, annotations)
+
+        findCyclicalLowerBound(tp) andAlso { sym =>
+          if (needsCycleCheck) {
+            // neg/t1224:  trait C[T] ; trait A { type T >: C[T] <: C[C[T]] }
+            // To avoid an infinite loop on the above, we cannot break all cycles
+            log(s"Reinitializing info of $sym to catch any genuine cycles")
+            sym reset sym.info
+            sym.initialize
+          }
+        }
 
-      val tp = typeSig(tree, annotations)
+        sym.setInfo(if (!sym.isJavaDefined) tp else RestrictJavaArraysMap(tp))
 
-      findCyclicalLowerBound(tp) andAlso { sym =>
         if (needsCycleCheck) {
-          // neg/t1224:  trait C[T] ; trait A { type T >: C[T] <: C[C[T]] }
-          // To avoid an infinite loop on the above, we cannot break all cycles
-          log(s"Reinitializing info of $sym to catch any genuine cycles")
-          sym reset sym.info
-          sym.initialize
+          log(s"Needs cycle check: ${sym.debugLocationString}")
+          if (!typer.checkNonCyclic(tree.pos, tp))
+            sym setInfo ErrorType
         }
-      }
 
-      sym.setInfo(if (!sym.isJavaDefined) tp else RestrictJavaArraysMap(tp))
-
-      if (needsCycleCheck) {
-        log(s"Needs cycle check: ${sym.debugLocationString}")
-        if (!typer.checkNonCyclic(tree.pos, tp))
-          sym setInfo ErrorType
+        validate(sym)
       }
-
-      validate(sym)
     }
 
-    def moduleClassTypeCompleter(tree: ModuleDef) = mkTypeCompleter(tree) { sym =>
-      val moduleSymbol = tree.symbol
-      assert(moduleSymbol.moduleClass == sym, moduleSymbol.moduleClass)
-      moduleSymbol.info // sets moduleClass info as a side effect.
+    def moduleClassTypeCompleter(tree: ModuleDef) = new ModuleClassTypeCompleter(tree)
+    class ModuleClassTypeCompleter(tree: ModuleDef) extends TypeCompleterBase(tree) {
+      override def completeImpl(sym: Symbol): Unit = {
+        val moduleSymbol = tree.symbol
+        assert(moduleSymbol.moduleClass == sym, moduleSymbol.moduleClass)
+        moduleSymbol.info // sets moduleClass info as a side effect.
+      }
     }
 
-
-    def importTypeCompleter(imp: Import) = mkTypeCompleter(imp) { sym =>
-      sym setInfo importSig(imp)
+    def importTypeCompleter(tree: Import) = new ImportTypeCompleter(tree)
+    class ImportTypeCompleter(imp: Import) extends TypeCompleterBase(imp) {
+      override def completeImpl(sym: Symbol): Unit = {
+        sym setInfo importSig(imp)
+      }
     }
 
     import AnnotationInfo.{mkFilter => annotationFilter}
 
-    def implicitFactoryMethodCompleter(tree: DefDef, classSym: Symbol, sigCompleter: LockingTypeCompleter) = mkTypeCompleter(tree) { methSym =>
-      sigCompleter.completeImpl(methSym)
-
-      val annotations = classSym.initialize.annotations
-
-      methSym setAnnotations (annotations filter annotationFilter(MethodTargetClass, defaultRetention = false))
-      classSym setAnnotations (annotations filter annotationFilter(ClassTargetClass, defaultRetention = true))
-    }
-
-    def caseApplyMethodCompleter(tree: DefDef, sigCompleter: LockingTypeCompleter) = mkTypeCompleter(tree) { methSym =>
-      sigCompleter.completeImpl(methSym)
+    def implicitFactoryMethodCompleter(tree: DefDef, classSym: Symbol) = new CompleterWrapper(completerOf(tree)) {
+      override def complete(methSym: Symbol): Unit = {
+        super.complete(methSym)
+        val annotations = classSym.initialize.annotations
 
-      // don't propagate e.g. @volatile annot to apply's argument
-      def retainOnlyParamAnnots(param: Symbol) =
-        param setAnnotations (param.annotations filter AnnotationInfo.mkFilter(ParamTargetClass, defaultRetention = false))
-
-      methSym.info.paramss.foreach(_.foreach(retainOnlyParamAnnots))
+        methSym setAnnotations (annotations filter annotationFilter(MethodTargetClass, defaultRetention = false))
+        classSym setAnnotations (annotations filter annotationFilter(ClassTargetClass, defaultRetention = true))
+      }
     }
 
     // complete the type of a value definition (may have a method symbol, for those valdefs that never receive a field,
     // as specified by Field.noFieldFor)
-    def valTypeCompleter(tree: ValDef) = mkTypeCompleter(tree) { fieldOrGetterSym =>
-      val mods = tree.mods
-      val isGetter = fieldOrGetterSym.isMethod
-      val annots =
-        if (mods.annotations.isEmpty) Nil
-        else {
-          val annotSigs = annotSig(mods.annotations)
+    def valTypeCompleter(tree: ValDef) = new ValTypeCompleter(tree)
+    class ValTypeCompleter(tree: ValDef) extends TypeCompleterBase(tree) {
+      override def completeImpl(fieldOrGetterSym: Symbol): Unit = {
+        val mods = tree.mods
+        val isGetter = fieldOrGetterSym.isMethod
+        val annots =
+          if (mods.annotations.isEmpty) Nil
+          else {
+            val annotSigs = annotSig(mods.annotations)
             if (isGetter) filterAccessorAnnots(annotSigs, tree) // if this is really a getter, retain annots targeting either field/getter
             else annotSigs filter annotationFilter(FieldTargetClass, !mods.isParamAccessor)
-        }
+          }
 
-      // must use typeSig, not memberSig (TODO: when do we need to switch namers?)
-      val sig = typeSig(tree, annots)
+        // must use typeSig, not memberSig (TODO: when do we need to switch namers?)
+        val sig = typeSig(tree, annots)
 
-      fieldOrGetterSym setInfo (if (isGetter) NullaryMethodType(sig) else sig)
+        fieldOrGetterSym setInfo (if (isGetter) NullaryMethodType(sig) else sig)
 
-      validate(fieldOrGetterSym)
+        validate(fieldOrGetterSym)
+      }
     }
 
     // knowing `isBean`, we could derive `isSetter` from `valDef.name`
-    def accessorTypeCompleter(valDef: ValDef, missingTpt: Boolean, isBean: Boolean, isSetter: Boolean) = mkTypeCompleter(valDef) { accessorSym =>
-      context.unit.synthetics get accessorSym match {
-        case Some(ddef: DefDef) =>
-          // `accessorSym` is the accessor for which we're completing the info (tree == ddef),
-          // while `valDef` is the field definition that spawned the accessor
-          // NOTE: `valTypeCompleter` handles abstract vals, trait vals and lazy vals, where the ValDef carries the getter's symbol
-
-          // reuse work done in valTypeCompleter if we already computed the type signature of the val
-          // (assuming the field and accessor symbols are distinct -- i.e., we're not in a trait)
-          val valSig =
-            if ((accessorSym ne valDef.symbol) && valDef.symbol.isInitialized) valDef.symbol.info
-            else typeSig(valDef, Nil) // don't set annotations for the valdef -- we just want to compute the type sig (TODO: dig deeper and see if we can use memberSig)
-
-          // patch up the accessor's tree if the valdef's tpt was not known back when the tree was synthesized
-          // can't look at `valDef.tpt` here because it may have been completed by now (this is why we pass in `missingTpt`)
-          // HACK: a param accessor `ddef.tpt.tpe` somehow gets out of whack with `accessorSym.info`, so always patch it back...
-          //       (the tpt is typed in the wrong namer, using the class as owner instead of the outer context, which is where param accessors should be typed)
-          if (missingTpt || accessorSym.isParamAccessor) {
-            if (!isSetter) ddef.tpt setType valSig
-            else if (ddef.vparamss.nonEmpty && ddef.vparamss.head.nonEmpty) ddef.vparamss.head.head.tpt setType valSig
-            else throw new TypeError(valDef.pos, s"Internal error: could not complete parameter/return type for $ddef from $accessorSym")
-          }
+    def accessorTypeCompleter(valDef: ValDef, missingTpt: Boolean, isBean: Boolean, isSetter: Boolean) = new AccessorTypeCompleter(valDef, missingTpt, isBean, isSetter)
+    class AccessorTypeCompleter(valDef: ValDef, missingTpt: Boolean, isBean: Boolean, isSetter: Boolean) extends TypeCompleterBase(valDef) {
+      override def completeImpl(accessorSym: Symbol): Unit = {
+        context.unit.synthetics get accessorSym match {
+          case Some(ddef: DefDef) =>
+            // `accessorSym` is the accessor for which we're completing the info (tree == ddef),
+            // while `valDef` is the field definition that spawned the accessor
+            // NOTE: `valTypeCompleter` handles abstract vals, trait vals and lazy vals, where the ValDef carries the getter's symbol
+
+            // reuse work done in valTypeCompleter if we already computed the type signature of the val
+            // (assuming the field and accessor symbols are distinct -- i.e., we're not in a trait)
+            val valSig =
+              if ((accessorSym ne valDef.symbol) && valDef.symbol.isInitialized) valDef.symbol.info
+              else typeSig(valDef, Nil) // don't set annotations for the valdef -- we just want to compute the type sig (TODO: dig deeper and see if we can use memberSig)
+
+            // patch up the accessor's tree if the valdef's tpt was not known back when the tree was synthesized
+            // can't look at `valDef.tpt` here because it may have been completed by now (this is why we pass in `missingTpt`)
+            // HACK: a param accessor `ddef.tpt.tpe` somehow gets out of whack with `accessorSym.info`, so always patch it back...
+            //       (the tpt is typed in the wrong namer, using the class as owner instead of the outer context, which is where param accessors should be typed)
+            if (missingTpt || accessorSym.isParamAccessor) {
+              if (!isSetter) ddef.tpt setType valSig
+              else if (ddef.vparamss.nonEmpty && ddef.vparamss.head.nonEmpty) ddef.vparamss.head.head.tpt setType valSig
+              else throw new TypeError(valDef.pos, s"Internal error: could not complete parameter/return type for $ddef from $accessorSym")
+            }
 
-          val mods = valDef.mods
-          val annots =
-            if (mods.annotations.isEmpty) Nil
-            else filterAccessorAnnots(annotSig(mods.annotations), valDef, isSetter, isBean)
+            val mods = valDef.mods
+            val annots =
+              if (mods.annotations.isEmpty) Nil
+              else filterAccessorAnnots(annotSig(mods.annotations), valDef, isSetter, isBean)
 
-          // for a setter, call memberSig to attribute the parameter (for a bean, we always use the regular method sig completer since they receive method types)
-          // for a regular getter, make sure it gets a NullaryMethodType (also, no need to recompute it: we already have the valSig)
-          val sig =
+            // for a setter, call memberSig to attribute the parameter (for a bean, we always use the regular method sig completer since they receive method types)
+            // for a regular getter, make sure it gets a NullaryMethodType (also, no need to recompute it: we already have the valSig)
+            val sig =
             if (isSetter || isBean) typeSig(ddef, annots)
             else {
               if (annots.nonEmpty) annotate(accessorSym, annots)
@@ -888,16 +945,17 @@ trait Namers extends MethodSynthesis {
               NullaryMethodType(valSig)
             }
 
-          accessorSym setInfo pluginsTypeSigAccessor(sig, typer, valDef, accessorSym)
+            accessorSym setInfo pluginsTypeSigAccessor(sig, typer, valDef, accessorSym)
 
-          if (!isBean && accessorSym.isOverloaded)
-            if (isSetter) ddef.rhs.setType(ErrorType)
-            else GetterDefinedTwiceError(accessorSym)
+            if (!isBean && accessorSym.isOverloaded)
+              if (isSetter) ddef.rhs.setType(ErrorType)
+              else GetterDefinedTwiceError(accessorSym)
 
-          validate(accessorSym)
+            validate(accessorSym)
 
-        case _ =>
-          throw new TypeError(valDef.pos, s"Internal error: no synthetic tree found for bean accessor $accessorSym")
+          case _ =>
+            throw new TypeError(valDef.pos, s"Internal error: no synthetic tree found for bean accessor $accessorSym")
+        }
       }
     }
 
@@ -942,11 +1000,14 @@ trait Namers extends MethodSynthesis {
     }
 
 
-    def selfTypeCompleter(tree: Tree) = mkTypeCompleter(tree) { sym =>
-      val selftpe = typer.typedType(tree).tpe
-      sym setInfo {
-        if (selftpe.typeSymbol isNonBottomSubClass sym.owner) selftpe
-        else intersectionType(List(sym.owner.tpe, selftpe))
+    def selfTypeCompleter(tree: Tree) = new SelfTypeCompleter(tree)
+    class SelfTypeCompleter(tree: Tree) extends TypeCompleterBase(tree) {
+      override def completeImpl(sym: Symbol): Unit = {
+        val selftpe = typer.typedType(tree).tpe
+        sym setInfo {
+          if (selftpe.typeSymbol isNonBottomSubClass sym.owner) selftpe
+          else intersectionType(List(sym.owner.tpe, selftpe))
+        }
       }
     }
 
@@ -1020,7 +1081,7 @@ trait Namers extends MethodSynthesis {
 
       val sym = (
         if (hasType || hasName) {
-          owner.typeOfThis = if (hasType) selfTypeCompleter(tpt) else owner.tpe_*
+          owner.typeOfThis = if (hasType) new SelfTypeCompleter(tpt) else owner.tpe_*
           val selfSym = owner.thisSym setPos self.pos
           if (hasName) selfSym setName name else selfSym
         }
@@ -1114,7 +1175,7 @@ trait Namers extends MethodSynthesis {
       val res = GenPolyType(tparams0, resultType)
       val pluginsTp = pluginsTypeSig(res, typer, cdef, WildcardType)
 
-      // Already assign the type to the class symbol (monoTypeCompleter will do it again).
+      // Already assign the type to the class symbol (MonoTypeCompleter will do it again).
       // Allows isDerivedValueClass to look at the info.
       clazz setInfo pluginsTp
       if (clazz.isDerivedValueClass) {
@@ -1128,7 +1189,7 @@ trait Namers extends MethodSynthesis {
 
     private def moduleSig(mdef: ModuleDef): Type = {
       val moduleSym = mdef.symbol
-      // The info of both the module and the moduleClass symbols need to be assigned. monoTypeCompleter assigns
+      // The info of both the module and the moduleClass symbols need to be assigned. MonoTypeCompleter assigns
       // the result of typeSig to the module symbol. The module class info is assigned here as a side-effect.
       val result = templateSig(mdef.impl)
       val pluginsTp = pluginsTypeSig(result, typer, mdef, WildcardType)
@@ -1464,7 +1525,7 @@ trait Namers extends MethodSynthesis {
 
             val defTpt =
               // don't mess with tpt's of case copy default getters, because assigning something other than TypeTree()
-              // will break the carefully orchestrated naming/typing logic that involves enterCopyMethod and caseClassCopyMeth
+              // will break the carefully orchestrated naming/typing logic that involves copyMethodCompleter and caseClassCopyMeth
               if (meth.isCaseCopy) TypeTree()
               else {
                 // If the parameter type mentions any type parameter of the method, let the compiler infer the
@@ -1528,7 +1589,7 @@ trait Namers extends MethodSynthesis {
                 // (a val's name ends in a " ", so can't compare to def)
                 val overridingSym = if (isGetter) vdef.symbol else vdef.symbol.getterIn(valOwner)
 
-                // We're called from an accessorTypeCompleter, which is completing the info for the accessor's symbol,
+                // We're called from an AccessorTypeCompleter, which is completing the info for the accessor's symbol,
                 // which may or may not be `vdef.symbol` (see isGetter above)
                 val overridden = safeNextOverriddenSymbol(overridingSym)
 
@@ -1671,7 +1732,7 @@ trait Namers extends MethodSynthesis {
     }
 
     /**
-     * TypeSig is invoked by monoTypeCompleters. It returns the type of a definition which
+     * TypeSig is invoked by MonoTypeCompleters. It returns the type of a definition which
      * is then assigned to the corresponding symbol (typeSig itself does not need to assign
      * the type to the symbol, but it can if necessary).
      */
@@ -1862,10 +1923,9 @@ trait Namers extends MethodSynthesis {
     }
   }
 
-  def mkTypeCompleter(t: Tree)(c: Symbol => Unit) = new LockingTypeCompleter with FlagAgnosticCompleter {
-    val tree = t
-    def completeImpl(sym: Symbol) = c(sym)
-  }
+  // NOTE: only meant for monomorphic definitions,
+  // do not use to wrap existing completers (see CompleterWrapper for that)
+  abstract class TypeCompleterBase[T <: Tree](val tree: T) extends LockingTypeCompleter with FlagAgnosticCompleter
 
   trait LockingTypeCompleter extends TypeCompleter {
     def completeImpl(sym: Symbol): Unit
@@ -1909,6 +1969,22 @@ trait Namers extends MethodSynthesis {
     }
   }
 
+  /**
+    * Wrap an existing completer to do some post/pre-processing of the completed type.
+    *
+    * @param completer
+    */
+  class CompleterWrapper(completer: TypeCompleter) extends TypeCompleter {
+    // override important when completer.isInstanceOf[PolyTypeCompleter]!
+    override val typeParams = completer.typeParams
+
+    val tree = completer.tree
+
+    override def complete(sym: Symbol): Unit = {
+      completer.complete(sym)
+    }
+  }
+
   // Can we relax these restrictions? For motivation, see
   //    test/files/pos/depmet_implicit_oopsla_session_2.scala
   //    neg/depmet_try_implicit.scala
diff --git a/src/compiler/scala/tools/nsc/typechecker/Typers.scala b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
index 85ca9950c4..05f7deb352 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Typers.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Typers.scala
@@ -3099,8 +3099,7 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
           || (looker.hasAccessorFlag && !accessed.hasAccessorFlag && accessed.isPrivate)
         )
 
-      def checkNoDoubleDefs: Unit = {
-        val scope = if (inBlock) context.scope else context.owner.info.decls
+      def checkNoDoubleDefs(scope: Scope): Unit = {
         var e = scope.elems
         while ((e ne null) && e.owner == scope) {
           var e1 = scope.lookupNextEntry(e)
@@ -3143,8 +3142,7 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
         }
       }
 
-      def addSynthetics(stats: List[Tree]): List[Tree] = {
-        val scope = if (inBlock) context.scope else context.owner.info.decls
+      def addSynthetics(stats: List[Tree], scope: Scope): List[Tree] = {
         var newStats = new ListBuffer[Tree]
         var moreToAdd = true
         while (moreToAdd) {
@@ -3219,11 +3217,14 @@ trait Typers extends Adaptations with Tags with TypersTracking with PatternTyper
       val stats1 = stats mapConserve typedStat
       if (phase.erasedTypes) stats1
       else {
+        val scope = if (inBlock) context.scope else context.owner.info.decls
+
         // As packages are open, it doesn't make sense to check double definitions here. Furthermore,
         // it is expensive if the package is large. Instead, such double definitions are checked in `Namers.enterInScope`
         if (!context.owner.isPackageClass)
-          checkNoDoubleDefs
-        addSynthetics(stats1)
+          checkNoDoubleDefs(scope)
+
+        addSynthetics(stats1, scope)
       }
     }
 
diff --git a/src/compiler/scala/tools/nsc/typechecker/Unapplies.scala b/src/compiler/scala/tools/nsc/typechecker/Unapplies.scala
index f2e9b260b0..c13257f6ec 100644
--- a/src/compiler/scala/tools/nsc/typechecker/Unapplies.scala
+++ b/src/compiler/scala/tools/nsc/typechecker/Unapplies.scala
@@ -24,7 +24,7 @@ trait Unapplies extends ast.TreeDSL {
   private def unapplyParamName = nme.x_0
   private def caseMods         = Modifiers(SYNTHETIC | CASE)
 
-  // In the typeCompleter (templateSig) of a case class (resp it's module),
+  // In the typeCompleter (templateSig) of a case class (resp its module),
   // synthetic `copy` (reps `apply`, `unapply`) methods are added. To compute
   // their signatures, the corresponding ClassDef is needed. During naming (in
   // `enterClassDef`), the case class ClassDef is added as an attachment to the
diff --git a/src/reflect/scala/reflect/internal/tpe/FindMembers.scala b/src/reflect/scala/reflect/internal/tpe/FindMembers.scala
index 6ba48cb44d..510d76793e 100644
--- a/src/reflect/scala/reflect/internal/tpe/FindMembers.scala
+++ b/src/reflect/scala/reflect/internal/tpe/FindMembers.scala
@@ -285,4 +285,18 @@ trait FindMembers {
       initBaseClasses.head.newOverloaded(tpe, members)
     }
   }
+
+  private[scala] final class HasMember(tpe: Type, name: Name, excludedFlags: Long, requiredFlags: Long) extends FindMemberBase[Boolean](tpe, name, excludedFlags, requiredFlags) {
+    private[this] var _result = false
+    override protected def result: Boolean = _result
+
+    protected def shortCircuit(sym: Symbol): Boolean = {
+      _result = true
+      true // prevents call to addMemberIfNew
+    }
+
+    // Not used
+    protected def addMemberIfNew(sym: Symbol): Unit = {}
+  }
+
 }
diff --git a/test/files/neg/userdefined_apply.check b/test/files/neg/userdefined_apply.check
new file mode 100644
index 0000000000..c8c8976f5f
--- /dev/null
+++ b/test/files/neg/userdefined_apply.check
@@ -0,0 +1,25 @@
+userdefined_apply.scala:3: error: overloaded method apply needs result type
+  private def apply(x: Int) = if (x > 0) new ClashOverloadNoSig(x) else apply("")
+                                                                             ^
+userdefined_apply.scala:14: error: overloaded method apply needs result type
+  private def apply(x: Int) = if (x > 0) ClashRecNoSig(1) else ???
+                                         ^
+userdefined_apply.scala:21: error: overloaded method apply needs result type
+  private def apply(x: Boolean) = if (x) NoClashNoSig(1) else ???
+                                         ^
+userdefined_apply.scala:28: error: overloaded method apply needs result type
+  private def apply(x: Boolean) = if (x) NoClashOverload(1) else apply("")
+                                                        ^
+userdefined_apply.scala:45: error: recursive method apply needs result type
+case class NoClashNoSigPoly private(x: Int)
+           ^
+userdefined_apply.scala:39: error: NoClashNoSigPoly.type does not take parameters
+  def apply(x: T) = if (???) NoClashNoSigPoly(1) else ???
+                                             ^
+userdefined_apply.scala:57: error: recursive method apply needs result type
+case class ClashNoSigPoly private(x: Int)
+           ^
+userdefined_apply.scala:51: error: ClashNoSigPoly.type does not take parameters
+  def apply(x: T) = if (???) ClashNoSigPoly(1) else ???
+                                           ^
+8 errors found
diff --git a/test/files/neg/userdefined_apply.scala b/test/files/neg/userdefined_apply.scala
new file mode 100644
index 0000000000..0a0d960b39
--- /dev/null
+++ b/test/files/neg/userdefined_apply.scala
@@ -0,0 +1,57 @@
+object ClashOverloadNoSig {
+  // error: overloaded method apply needs result type
+  private def apply(x: Int) = if (x > 0) new ClashOverloadNoSig(x) else apply("")
+
+  def apply(x: String): ClashOverloadNoSig = ???
+}
+
+case class ClashOverloadNoSig private(x: Int)
+
+object ClashRecNoSig {
+  // TODO: status quo is that the error refers to an overloaded method, which is actually recursive
+  // (we should have unlinked the symbol in the `if(suppress)` part of `applyUnapplyMethodCompleter`)
+  // error: recursive method apply needs result type
+  private def apply(x: Int) = if (x > 0) ClashRecNoSig(1) else ???
+}
+
+case class ClashRecNoSig private(x: Int)
+
+object NoClashNoSig {
+  // error: overloaded method apply needs result type
+  private def apply(x: Boolean) = if (x) NoClashNoSig(1) else ???
+}
+
+case class NoClashNoSig private(x: Int)
+
+object NoClashOverload {
+  // error: overloaded method apply needs result type
+  private def apply(x: Boolean) = if (x) NoClashOverload(1) else apply("")
+
+  def apply(x: String): NoClashOverload = ???
+}
+
+case class NoClashOverload private(x: Int)
+
+
+class BaseNCNSP[T] {
+  // TODO: suppress the following error
+  // error: NoClashNoSigPoly.type does not take parameters
+  def apply(x: T) = if (???) NoClashNoSigPoly(1) else ???
+}
+
+object NoClashNoSigPoly extends BaseNCNSP[Boolean]
+// TODO: position error at definition of apply in superclass instead of on case clss
+// error: recursive method apply needs result type
+case class NoClashNoSigPoly private(x: Int)
+
+
+class BaseCNSP[T] {
+  // TODO: suppress the following error
+  // error: ClashNoSigPoly.type does not take parameters
+  def apply(x: T) = if (???) ClashNoSigPoly(1) else ???
+}
+
+object ClashNoSigPoly extends BaseCNSP[Int]
+// TODO: position error at definition of apply in superclass instead of on case clss
+// error: recursive method apply needs result type
+case class ClashNoSigPoly private(x: Int)
diff --git a/test/files/pos/userdefined_apply.scala b/test/files/pos/userdefined_apply.scala
new file mode 100644
index 0000000000..e29f9f5141
--- /dev/null
+++ b/test/files/pos/userdefined_apply.scala
@@ -0,0 +1,54 @@
+// NOTE: the companion inherits a public apply method from Function1!
+case class NeedsCompanion private (x: Int)
+
+object ClashNoSig { // ok
+  private def apply(x: Int) = if (x > 0) new ClashNoSig(x) else ???
+}
+case class ClashNoSig private (x: Int)
+
+
+object Clash {
+  private def apply(x: Int) = if (x > 0) new Clash(x) else ???
+}
+case class Clash private (x: Int)
+
+object ClashSig {
+  private def apply(x: Int): ClashSig = if (x > 0) new ClashSig(x) else ???
+}
+case class ClashSig private (x: Int)
+
+object ClashOverload {
+  private def apply(x: Int): ClashOverload = if (x > 0) new ClashOverload(x) else apply("")
+  def apply(x: String): ClashOverload = ???
+}
+case class ClashOverload private (x: Int)
+
+object NoClashSig {
+  private def apply(x: Boolean): NoClashSig = if (x) NoClashSig(1) else ???
+}
+case class NoClashSig private (x: Int)
+
+object NoClashOverload {
+  // needs full sig
+  private def apply(x: Boolean): NoClashOverload = if (x) NoClashOverload(1) else apply("")
+  def apply(x: String): NoClashOverload = ???
+}
+case class NoClashOverload private (x: Int)
+
+
+
+class BaseNCP[T] {
+  // error: overloaded method apply needs result type
+  def apply(x: T): NoClashPoly = if (???) NoClashPoly(1) else ???
+}
+
+object NoClashPoly extends BaseNCP[Boolean]
+case class NoClashPoly private(x: Int)
+
+
+class BaseCP[T] {
+  // error: overloaded method apply needs result type
+  def apply(x: T): ClashPoly = if (???) ClashPoly(1) else ???
+}
+object ClashPoly extends BaseCP[Int]
+case class ClashPoly private(x: Int)
diff --git a/test/files/pos/userdefined_apply_poly_overload.scala b/test/files/pos/userdefined_apply_poly_overload.scala
new file mode 100644
index 0000000000..6760c1424f
--- /dev/null
+++ b/test/files/pos/userdefined_apply_poly_overload.scala
@@ -0,0 +1,13 @@
+object Foo {
+  // spurious error if:
+  //   - this definition precedes that of apply (which is overloaded with the synthetic one derived from the case class)
+  //   - AND `Foo.apply` is explicitly applied to `[A]` (no error if `[A]` is inferred)
+  //
+  def referToPolyOverloadedApply[A]: Foo[A] = Foo.apply[A]("bla")
+  //                                                   ^
+  //  found   : String("bla")
+  //  required: Int
+
+  def apply[A](x: Int): Foo[A] = ???
+}
+case class Foo[A](x: String) // must be polymorphic