SI-5189 1/2: inferConstrInst uses correct variance

fixed concurrent.impl.Promise by making FState invariant
(it would be unsound to make it covariant)

Author: Adriaan Moors

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

@@ -1068,7 +1068,7 @@ trait Infer {
           try {
             // debuglog("TVARS "+ (tvars map (_.constr)))
             // look at the argument types of the primary constructor corresponding to the pattern
-            val variances  = undetparams map varianceInType(ctorTp)
+            val variances  = undetparams map varianceInType(ctorTp.paramTypes.headOption getOrElse ctorTp)
             val targs      = solvedTypes(tvars, undetparams, variances, true, lubDepth(List(resTp, pt)))
             // checkBounds(tree, NoPrefix, NoSymbol, undetparams, targs, "inferred ")
             // no checkBounds here. If we enable it, test bug602 fails.

src/library/scala/concurrent/impl/Promise.scala

@@ -80,8 +80,10 @@ object Promise {
   def EmptyPending[T](): FState[T] = emptyPendingValue.asInstanceOf[FState[T]]
 
   /** Represents the internal state.
+   *
+   * [adriaan] it's unsound to make FState covariant (tryComplete won't type check)
    */
-  sealed trait FState[+T] { def value: Option[Try[T]] }
+  sealed trait FState[T] { def value: Option[Try[T]] }
 
   case class Pending[T](listeners: List[Try[T] => Any] = Nil) extends FState[T] {
     def value: Option[Try[T]] = None
@@ -155,7 +157,11 @@ object Promise {
           def tryComplete(v: Try[T]): List[Try[T] => Any] = {
             getState match {
               case cur @ Pending(listeners) =>
-                if (updateState(cur, if (v.isFailure) Failure(Some(v.asInstanceOf[util.Failure[T]])) else Success(Some(v.asInstanceOf[util.Success[T]])))) listeners
+                val newState =
+                  if (v.isFailure) Failure(Some(v.asInstanceOf[util.Failure[T]]))
+                  else Success(Some(v.asInstanceOf[util.Success[T]]))
+
+                if (updateState(cur, newState)) listeners
                 else tryComplete(v)
               case _ => null
             }

test/files/neg/t5189.check

@@ -0,0 +1,6 @@
+t5189.scala:3: error: type mismatch;
+ found   : Nothing => Any
+ required: Any => Any
+  def f(x: Any): Any => Any = x match { case Foo(bar) => bar }
+                                                         ^
+one error found

test/files/neg/t5189.scala

@@ -0,0 +1,5 @@
+class TestNeg1 {
+  case class Foo[T, U](f: T => U)
+  def f(x: Any): Any => Any = x match { case Foo(bar) => bar }
+  // uh-oh, Any => Any should be Nothing => Any.
+}