Disallow hk type parameters in lower bounds.

Also: various cleanups to comments.

Author: odersky

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

@@ -550,7 +550,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
    *
    *   (2) Try to eta expand the constructor of `other`.
    *
-   *   (3a) In mode `TypeVarsMissConetxt` replace the projection's hk constructor parameter
+   *   (3a) In mode `TypevarsMissConetxt` replace the projection's hk constructor parameter
    *        by the eta expansion of step (2) reapplied to the projection's arguments.
    *   (3b) In normal mode, try to unify the projection's hk constructor parameter with
    *        the eta expansion of step(2)
@@ -573,7 +573,7 @@ class TypeComparer(initctx: Context) extends DotClass with ConstraintHandling {
                   else
                     (tryInstantiate(param, EtaExpansion(tycon)), projection)
                 ok &&
-                (if (inOrder) isSubType(projection1, other) else isSubType(other, projection1)) // ### move out?
+                (if (inOrder) isSubType(projection1, other) else isSubType(other, projection1))
               case _ =>
                 false
             }

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

@@ -613,12 +613,8 @@ trait Applications extends Compatibility { self: Typer =>
     assignType(cpy.TypeApply(tree)(typedFn, typedArgs), typedFn, typedArgs)
   }
 
-  def adaptTypeArg(tree: tpd.Tree, bound: Type)(implicit ctx: Context): tpd.Tree = {
-    //val was = tree.tpe.EtaExpandIfHK(bound)
-    //val now = tree.tpe.adaptIfHK(bound) // ###
-    //if (was != now) println(i"diff adapt ${tree.tpe} to $bound, was: $was, now: $now")
+  def adaptTypeArg(tree: tpd.Tree, bound: Type)(implicit ctx: Context): tpd.Tree =
     tree.withType(tree.tpe.adaptIfHK(bound))
-  }
 
   /** Rewrite `new Array[T](....)` trees to calls of newXYZArray methods. */
   def convertNewArray(tree: tpd.Tree)(implicit ctx: Context): tpd.Tree = tree match {

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

@@ -401,6 +401,17 @@ trait Checking {
       errorTree(tpt, d"missing type parameter for ${tpt.tpe}")
     }
     else tpt
+
+  def checkLowerNotHK(sym: Symbol, tparams: List[Symbol], pos: Position)(implicit ctx: Context) =
+    if (tparams.nonEmpty)
+      sym.info match {
+        case info: TypeAlias => // ok
+        case TypeBounds(lo, _) =>
+          for (tparam <- tparams)
+            if (tparam.typeRef.occursIn(lo))
+              ctx.error(i"type parameter ${tparam.name} may not occur in lower bound $lo", pos)
+        case _ =>
+      }
 }
 
 trait NoChecking extends Checking {
@@ -414,4 +425,5 @@ trait NoChecking extends Checking {
   override def checkNoDoubleDefs(cls: Symbol)(implicit ctx: Context): Unit = ()
   override def checkParentCall(call: Tree, caller: ClassSymbol)(implicit ctx: Context) = ()
   override def checkSimpleKinded(tpt: Tree)(implicit ctx: Context): Tree = tpt
+  override def checkLowerNotHK(sym: Symbol, tparams: List[Symbol], pos: Position)(implicit ctx: Context) = ()
 }

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

@@ -845,7 +845,6 @@ class Namer { typer: Typer =>
    *  of arguments in F-bounds, because the recursive type was initialized with
    *  TypeBounds.empty.
    */
-  // ### Check whether this is still needed!
   def etaExpandArgs(implicit ctx: Context) = new TypeMap {
     def apply(tp: Type): Type = tp match {
       case tp: RefinedType =>

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

@@ -930,6 +930,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
 
   def typedTypeDef(tdef: untpd.TypeDef, sym: Symbol)(implicit ctx: Context): Tree = track("typedTypeDef") {
     val TypeDef(name, rhs) = tdef
+    checkLowerNotHK(sym, tdef.tparams.map(symbolOfTree), tdef.pos)
     completeAnnotations(tdef, sym)
     val _ = typedType(rhs) // unused, typecheck only to remove from typedTree
     assignType(cpy.TypeDef(tdef)(name, TypeTree(sym.info), Nil), sym)

test/dotc/tests.scala

@@ -147,6 +147,7 @@ class tests extends CompilerTest {
   @Test def neg_cycles = compileFile(negDir, "cycles", xerrors = 8)
   @Test def neg_boundspropagation = compileFile(negDir, "boundspropagation", xerrors = 5)
   @Test def neg_refinedSubtyping = compileFile(negDir, "refinedSubtyping", xerrors = 2)
+  @Test def neg_hklower = compileFile(negDir, "hklower", xerrors = 3)
   @Test def neg_i0091_infpaths = compileFile(negDir, "i0091-infpaths", xerrors = 3)
   @Test def neg_i0248_inherit_refined = compileFile(negDir, "i0248-inherit-refined", xerrors = 4)
   @Test def neg_i0281 = compileFile(negDir, "i0281-null-primitive-conforms", xerrors = 3)

tests/neg/hklower.scala

@@ -0,0 +1,11 @@
+class Test {
+
+  type T[X] // OK
+  type U[X] = T[X] // OK
+
+  type V[X] >: T[X] // error
+  type W[X] >: T[X] <: T[X] // error
+
+  def f[C[X] >: T[X]]() = ??? // error
+
+}