Fix #779 recursive hk types

src/dotty/tools/dotc/core/TypeApplications.scala

@@ -188,7 +188,17 @@ class TypeApplications(val self: Type) extends AnyVal {
     if (args.isEmpty || ctx.erasedTypes) self
     else {
       val res = instantiate(self, self)
-      if (res.isInstantiatedLambda) res.select(tpnme.Apply) else res
+      if (res.isInstantiatedLambda)
+        // Note: isInstantiatedLambda needs to be conservative, using isSafeLambda
+        // in order to avoid cyclic reference errors. But this means that some fully
+        // instantiated types will remain unprojected, which essentially means
+        // that they stay as higher-kinded types. checkNonCyclic checks the type again
+        // and potentially inserts an #Apply then. Hopefully, this catches all types
+        // that fall through the hole. Not adding an #Apply typically manifests itself
+        // with a <:< failure of two types that "look the same". An example is #779,
+        // where compiling scala.immutable.Map gives a bounds violation.
+        res.select(tpnme.Apply)
+      else res
     }
   }
 

src/dotty/tools/dotc/core/TypeOps.scala

@@ -43,7 +43,7 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
    *  for more complicated types.
    */
   final def asSeenFrom(tp: Type, pre: Type, cls: Symbol): Type = {
-    val m = if (pre.isStable || !ctx.phase.isTyper) null else new AsSeenFromMap(pre, cls)
+    val m = if (isLegalPrefix(pre)) null else new AsSeenFromMap(pre, cls)
     var res = asSeenFrom(tp, pre, cls, m)
     if (m != null && m.unstable) asSeenFrom(tp, SkolemType(pre), cls) else res
   }
@@ -61,7 +61,7 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
       if ((pre eq NoType) || (pre eq NoPrefix) || (cls is PackageClass))
         tp
       else if (thiscls.derivesFrom(cls) && pre.baseTypeRef(thiscls).exists) {
-        if (theMap != null && theMap.currentVariance <= 0 && !pre.isStable)
+        if (theMap != null && theMap.currentVariance <= 0 && !isLegalPrefix(pre))
           theMap.unstable = true
         pre match {
           case SuperType(thispre, _) => thispre
@@ -111,6 +111,9 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
     }
   }
 
+  private def isLegalPrefix(pre: Type)(implicit ctx: Context) =
+    pre.isStable || !ctx.phase.isTyper
+
   /** The TypeMap handling the asSeenFrom in more complicated cases */
   class AsSeenFromMap(pre: Type, cls: Symbol) extends TypeMap {
     def apply(tp: Type) = asSeenFrom(tp, pre, cls, this)

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

@@ -115,8 +115,18 @@ object Checking {
         val parent1 = this(parent)
         val saved = cycleOK
         cycleOK = nestedCycleOK
-        try tp.derivedRefinedType(parent1, name, this(tp.refinedInfo))
-        finally cycleOK = saved
+
+        /** A derived refined type with two possible tweaks:
+         *  (1) LazyRefs in parents are pulled out,
+         *  (2) #Apply is added if the type is a fully applied type lambda.
+         */
+        def derivedType(p: Type): Type = p match {
+          case p: LazyRef => LazyRef(() => derivedType(p.ref))
+          case _ =>
+            val res = tp.derivedRefinedType(p, name, this(tp.refinedInfo))
+            if (res.isSafeLambda && res.typeParams.isEmpty) res.select(tpnme.Apply) else res
+        }
+        try derivedType(parent1) finally cycleOK = saved
       case tp @ TypeRef(pre, name) =>
         try {
           // A prefix is interesting if it might contain (transitively) a reference
@@ -130,6 +140,9 @@ object Checking {
             case SuperType(thistp, _) => isInteresting(thistp)
             case AndType(tp1, tp2) => isInteresting(tp1) || isInteresting(tp2)
             case OrType(tp1, tp2) => isInteresting(tp1) && isInteresting(tp2)
+            case _: RefinedType => false
+              // Note: it's important not to visit parents of RefinedTypes,
+              // since otherwise spurious #Apply projections might be inserted.
             case _ => false
           }
           // If prefix is interesting, check info of typeref recursively, marking the referred symbol
@@ -158,6 +171,9 @@ object Checking {
    *  @pre     sym is not yet initialized (i.e. its type is a Completer).
    *  @return  `info` where every legal F-bounded reference is proctected
    *                  by a `LazyRef`, or `ErrorType` if a cycle was detected and reported.
+   *                  Furthermore: Add an #Apply to a fully instantiated type lambda, if none was
+   *                  given before. This is necessary here because sometimes type lambdas are not
+   *                  recognized when they are first formed.
    */
   def checkNonCyclic(sym: Symbol, info: Type, reportErrors: Boolean)(implicit ctx: Context): Type = {
     val checker = new CheckNonCyclicMap(sym, reportErrors)(ctx.addMode(Mode.CheckCyclic))