Avoid inserting multiple .apply's.

This can lead to stackoverflow, as i1639.scala shows.

Fixes #1639.

Author: odersky

src/dotty/tools/dotc/typer/ReTyper.scala

@@ -73,8 +73,8 @@ class ReTyper extends Typer {
 
   override def index(trees: List[untpd.Tree])(implicit ctx: Context) = ctx
 
-  override def tryInsertApplyOrImplicit(tree: Tree, pt: ProtoType)(fallBack: (Tree, TyperState) => Tree)(implicit ctx: Context): Tree =
-    fallBack(tree, ctx.typerState)
+  override def tryInsertApplyOrImplicit(tree: Tree, pt: ProtoType)(fallBack: => Tree)(implicit ctx: Context): Tree =
+    fallBack
 
   override def completeAnnotations(mdef: untpd.MemberDef, sym: Symbol)(implicit ctx: Context): Unit = ()
 

src/dotty/tools/dotc/typer/Typer.scala

@@ -1565,18 +1565,34 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
    *  `fallBack`.
    *
    *  1st strategy: Try to insert `.apply` so that the result conforms to prototype `pt`.
+   *                This strategy is not tried if the prototype represents already
+   *                another `.apply` or `.apply()` selection.
    *  2nd strategy: If tree is a select `qual.name`, try to insert an implicit conversion
    *    around the qualifier part `qual` so that the result conforms to the expected type
    *    with wildcard result type.
    */
-  def tryInsertApplyOrImplicit(tree: Tree, pt: ProtoType)(fallBack: (Tree, TyperState) => Tree)(implicit ctx: Context): Tree =
-    tryEither { implicit ctx =>
+  def tryInsertApplyOrImplicit(tree: Tree, pt: ProtoType)(fallBack: => Tree)(implicit ctx: Context): Tree = {
+
+    /** Is `pt` a prototype of an `apply` selection, or a parameterless function yielding one? */
+    def isApplyProto(pt: Type): Boolean = pt match {
+      case pt: SelectionProto => pt.name == nme.apply
+      case pt: FunProto => pt.args.isEmpty && isApplyProto(pt.resultType)
+      case pt: IgnoredProto => isApplyProto(pt.ignored)
+      case _ => false
+    }
+
+    def tryApply(implicit ctx: Context) = {
       val sel = typedSelect(untpd.Select(untpd.TypedSplice(tree), nme.apply), pt)
       if (sel.tpe.isError) sel else adapt(sel, pt)
-    } { (failedTree, failedState) =>
-      tryInsertImplicitOnQualifier(tree, pt).getOrElse(fallBack(failedTree, failedState))
     }
 
+    def tryImplicit =
+      tryInsertImplicitOnQualifier(tree, pt).getOrElse(fallBack)
+
+    if (isApplyProto(pt)) tryImplicit
+    else tryEither(tryApply(_))((_, _) => tryImplicit) 
+  }
+
   /** If this tree is a select node `qual.name`, try to insert an implicit conversion
    *  `c` around `qual` so that `c(qual).name` conforms to `pt`.
    */
@@ -1668,7 +1684,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
           def hasEmptyParams(denot: SingleDenotation) = denot.info.paramTypess == ListOfNil
           pt match {
             case pt: FunProto =>
-              tryInsertApplyOrImplicit(tree, pt)((_, _) => noMatches)
+              tryInsertApplyOrImplicit(tree, pt)(noMatches)
             case _ =>
               if (altDenots exists (_.info.paramTypess == ListOfNil))
                 typed(untpd.Apply(untpd.TypedSplice(tree), Nil), pt)
@@ -1707,7 +1723,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
           case Apply(_, _) => " more"
           case _ => ""
         }
-        (_, _) => errorTree(tree, em"$methodStr does not take$more parameters")
+        errorTree(tree, em"$methodStr does not take$more parameters")
       }
     }
 
@@ -1924,9 +1940,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
             case pt: FunProto =>
               adaptToArgs(wtp, pt)
             case pt: PolyProto =>
-              tryInsertApplyOrImplicit(tree, pt) {
-                (_, _) => tree // error will be reported in typedTypeApply
-              }
+              tryInsertApplyOrImplicit(tree, pt)(tree) // error will be reported in typedTypeApply
             case _ =>
               if (ctx.mode is Mode.Type) adaptType(tree.tpe)
               else adaptNoArgs(wtp)

tests/neg/i1639.scala

@@ -0,0 +1,10 @@
+class Bar {
+  implicit def f(implicit x: String): String = x
+
+  implicitly[String](f) // error: divergent (turn -explaintypes on to see it)
+}
+
+class Foo(implicit val bar: String) {
+  def this() = this("baz") // error: none of the alternatives match arguments
+}
+