Allow user-defined [un]apply in case companion

Don't emit a synthetic `apply` (or `unapply`) when it would
clash with an existing one. This allows e.g., a `private apply`,
along with a `case class` with a `private` constructor.

We have to retract the synthetic method in a pretty roundabout way,
as we need the other methods and the owner to be completed already.
Unless we have to complete the synthetic `apply` while completing
the user-defined one, this should not be a problem. If this does
happen, this implies there's a cycle in computing the user-defined
signature and the synthetic one, which is not allowed.

src/compiler/scala/tools/nsc/typechecker/Namers.scala

@@ -610,7 +610,15 @@ trait Namers extends MethodSynthesis {
       noDuplicates(selectors map (_.rename), AppearsTwice)
     }
 
-    def enterCopyMethod(copyDef: DefDef): Symbol = {
+    class CompleterWrapper(completer: TypeCompleter) extends TypeCompleter {
+      val tree = completer.tree
+
+      override def complete(sym: Symbol): Unit = {
+        completer.complete(sym)
+      }
+    }
+
+    def copyMethodCompleter(copyDef: DefDef): TypeCompleter = {
       val sym      = copyDef.symbol
       val lazyType = completerOf(copyDef)
 
@@ -629,14 +637,57 @@ trait Namers extends MethodSynthesis {
         )
       }
 
-      sym setInfo {
-        mkTypeCompleter(copyDef) { sym =>
-          assignParamTypes()
-          lazyType complete sym
-        }
+      mkTypeCompleter(copyDef) { sym =>
+        assignParamTypes()
+        lazyType 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 = {
+          super.complete(sym)
+
+          // If there's a same-named locked symbol, we're currently completing its signature.
+          // This means it (may) refer to us, and is thus either overloaded or recursive without a signature.
+          // rule out locked symbols from the owner.info.member call
+          val scopePartiallyCompleted =
+            companionContext.scope.lookupAll(sym.name).exists(existing => existing != sym && existing.hasFlag(LOCKED))
+
+          val suppress =
+            scopePartiallyCompleted || {
+            val userDefined = companionContext.owner.info.member(sym.name).filter(_ != sym)
+            (userDefined != NoSymbol) && {
+              userDefined.info match {
+                // TODO: do we have something for this already? the synthetic symbol can't be overloaded, right?
+                case OverloadedType(pre, alternatives) =>
+                  // pre probably relevant because of inherited overloads?
+                  alternatives.exists(_.isErroneous) || alternatives.exists(alt => pre.memberInfo(alt) matches pre.memberInfo(sym))
+                case tp =>
+                  (tp eq ErrorType) || tp.matches(sym.info)
+              }
+            }
+          }
+
+          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: Tree): TypeCompleter = {
       val mono = namerOf(tree.symbol) monoTypeCompleter tree
       val tparams = treeInfo.typeParameters(tree)
@@ -697,11 +748,17 @@ trait Namers extends MethodSynthesis {
         val bridgeFlag = if (mods hasAnnotationNamed tpnme.bridgeAnnot) BRIDGE | ARTIFACT else 0
         val sym = assignAndEnterSymbol(tree) setFlag bridgeFlag
 
-        if (name == nme.copy && sym.isSynthetic)
-          enterCopyMethod(tree)
-        else
-          sym setInfo completerOf(tree)
-    }
+        // copy/apply/unapply synthetics are added using the addIfMissing mechanism,
+        // which ensures the owner has its preliminary info (we may add another decl here)
+        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
@@ -1351,7 +1408,7 @@ 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

test/files/neg/userdefined_apply.check

@@ -0,0 +1,13 @@
+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:12: error: overloaded method apply needs result type
+  private def apply(x: Int) = if (x > 0) ClashRecNoSig(1) else ???
+                                         ^
+userdefined_apply.scala:19: error: overloaded method apply needs result type
+  private def apply(x: Boolean) = if (x) NoClashNoSig(1) else ???
+                                         ^
+userdefined_apply.scala:26: error: overloaded method apply needs result type
+  private def apply(x: Boolean) = if (x) NoClashOverload(1) else apply("")
+                                                        ^
+four errors found

test/files/neg/userdefined_apply.scala

@@ -0,0 +1,31 @@
+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 {
+  // 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)

test/files/pos/userdefined_apply.scala

@@ -0,0 +1,36 @@
+// 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)