Closed 6029 ...

... in a less nice way than I would like. Essentially, we mask type errors at later stages that arise from comparing existentials and skolems because we know that they are not tracked correctly through all tree transformations. Never mind that all these types are going erased anyway shortly afterwards. It does not smell nice. But as I write in the comment, maybe the best way out is to
avoid skolems altogether. Such a change by far exceeds the scope of this pull request however.

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

@@ -1194,30 +1194,46 @@ trait Typers extends Modes with Adaptations with Tags {
               }
 
               val found = tree.tpe
-              val req = pt
-              if (!found.isErroneous && !req.isErroneous) {
-                if (!context.reportErrors && isPastTyper && req.skolemsExceptMethodTypeParams.nonEmpty) {
-                  // Ignore type errors raised in later phases that are due to mismatching types with existential skolems
-                  // We have lift crashing in 2.9 with an adapt failure in the pattern matcher.
-                  // Here's my hypothsis why this happens. The pattern matcher defines a variable of type
-                  //
-                  //   val x: T = expr
-                  //
-                  // where T is the type of expr, but T contains existential skolems ts.
-                  // In that case, this value definition does not typecheck.
-                  // The value definition
-                  //
-                  //   val x: T forSome { ts } = expr
-                  //
-                  // would typecheck. Or one can simply leave out the type of the `val`:
-                  //
-                  //   val x = expr
-                  context.unit.warning(tree.pos, "recovering from existential Skolem type error in tree \n" + tree + "\nwith type " + tree.tpe + "\n expected type = " + pt + "\n context = " + context.tree)
-                  adapt(tree, mode, deriveTypeWithWildcards(pt.skolemsExceptMethodTypeParams)(pt))
-                } else {
-                  // create an actual error
-                  AdaptTypeError(tree, found, req)
+              if (!found.isErroneous && !pt.isErroneous) {
+                if (!context.reportErrors && isPastTyper) {
+                  val (bound, req) = pt match {
+                    case ExistentialType(qs, tpe) => (qs, tpe)
+                    case _ => (Nil, pt)
+                  }
+                  val boundOrSkolems = bound ++ pt.skolemsExceptMethodTypeParams
+                  if (boundOrSkolems.nonEmpty) {
+                    // Ignore type errors raised in later phases that are due to mismatching types with existential skolems
+                    // We have lift crashing in 2.9 with an adapt failure in the pattern matcher.
+                    // Here's my hypothsis why this happens. The pattern matcher defines a variable of type
+                    //
+                    //   val x: T = expr
+                    //
+                    // where T is the type of expr, but T contains existential skolems ts.
+                    // In that case, this value definition does not typecheck.
+                    // The value definition
+                    //
+                    //   val x: T forSome { ts } = expr
+                    //
+                    // would typecheck. Or one can simply leave out the type of the `val`:
+                    //
+                    //   val x = expr
+                    //
+                    // SI-6029 shows another case where we also fail (in uncurry), but this time the expected
+                    // type is an existential type.
+                    //
+                    // The reason for both failures have to do with the way we (don't) transform
+                    // skolem types along with the trees that contain them. We'd need a
+                    // radically different approach to do it. But before investing a lot of time to
+                    // to do this (I have already sunk 3 full days with in the end futile attempts
+                    // to consistently transform skolems and fix 6029), I'd like to
+                    // investigate ways to avoid skolems completely.
+                    //
+                    log("recovering from existential or skolem type error in tree \n" + tree + "\nwith type " + tree.tpe + "\n expected type = " + pt + "\n context = " + context.tree)
+                    return adapt(tree, mode, deriveTypeWithWildcards(boundOrSkolems)(pt))
+                  }
                 }
+                // create an actual error
+                AdaptTypeError(tree, found, pt)
               }
               setError(tree)
             }
@@ -4531,7 +4547,7 @@ trait Typers extends Modes with Adaptations with Tags {
           assert(errorContainer == null, "Cannot set ambiguous error twice for identifier")
           errorContainer = tree
         }
-        
+
         val fingerPrint: Long = name.fingerPrint
 
         var defSym: Symbol = tree.symbol  // the directly found symbol

test/files/pos/t6029.scala

@@ -0,0 +1,3 @@
+final case class V[A](x: A) extends AnyVal {
+  def flatMap[B](f: A => V[B]) = if (true) this else f(x)
+}