Fix #2152: Instantiate dependent result type parameters

#2152 shows that dependent result type parameters can end up in
the types of terms, so we have to eliminate them. If we don't we
get orphan parameters in pickling.
Fix method name and comment

5 files changed, 22 insertions, 10 deletions

diff --git a/compiler/src/dotty/tools/dotc/core/NameKinds.scala b/compiler/src/dotty/tools/dotc/core/NameKinds.scala
index 0f08e4701..81ac3a02f 100644
--- a/compiler/src/dotty/tools/dotc/core/NameKinds.scala
+++ b/compiler/src/dotty/tools/dotc/core/NameKinds.scala
@@ -209,7 +209,7 @@ object NameKinds {
   val InlineAccessorName      = new UniqueNameKind("$_inlineAccessor_$")
   val TempResultName          = new UniqueNameKind("ev$")
   val EvidenceParamName       = new UniqueNameKind("evidence$")
-  val DepParamName            = new UniqueNameKind("<param>")
+  val DepParamName            = new UniqueNameKind("(param)")
   val LazyImplicitName        = new UniqueNameKind("$_lazy_implicit_$")
   val LazyLocalName           = new UniqueNameKind("$lzy")
   val LazyLocalInitName       = new UniqueNameKind("$lzyINIT")
diff --git a/compiler/src/dotty/tools/dotc/core/TypeOps.scala b/compiler/src/dotty/tools/dotc/core/TypeOps.scala
index 9593bfe93..0de823e97 100644
--- a/compiler/src/dotty/tools/dotc/core/TypeOps.scala
+++ b/compiler/src/dotty/tools/dotc/core/TypeOps.scala
@@ -7,6 +7,7 @@ import SymDenotations._, Denotations.SingleDenotation
 import config.Printers.typr
 import util.Positions._
 import NameOps._
+import NameKinds.DepParamName
 import Decorators._
 import StdNames._
 import Annotations._
@@ -169,6 +170,9 @@ trait TypeOps { this: Context => // TODO: Make standalone object.
       simplify(l, theMap) & simplify(r, theMap)
     case OrType(l, r) =>
       simplify(l, theMap) | simplify(r, theMap)
+    case tp: TypeVar if tp.origin.paramName.is(DepParamName) =>
+      val effectiveVariance = if (theMap == null) 1 else theMap.variance
+      tp.instanceOpt orElse tp.instantiate(fromBelow = effectiveVariance != -1)
     case _ =>
       (if (theMap != null) theMap else new SimplifyMap).mapOver(tp)
   }
diff --git a/compiler/src/dotty/tools/dotc/core/Types.scala b/compiler/src/dotty/tools/dotc/core/Types.scala
index c8c1886cc..955a5a11c 100644
--- a/compiler/src/dotty/tools/dotc/core/Types.scala
+++ b/compiler/src/dotty/tools/dotc/core/Types.scala
@@ -3509,7 +3509,7 @@ object Types {
 
     def apply(tp: Type): Type
 
-    protected var variance = 1
+    protected[core] var variance = 1
 
     protected def derivedSelect(tp: NamedType, pre: Type): Type =
       tp.derivedSelect(pre)
diff --git a/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala b/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala
index 398a7a17e..5d8240362 100644
--- a/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala
+++ b/compiler/src/dotty/tools/dotc/typer/ProtoTypes.scala
@@ -377,9 +377,10 @@ object ProtoTypes {
    *  Also, if `owningTree` is non-empty, add a type variable for each parameter.
    *  @return  The added type lambda, and the list of created type variables.
    */
-  def constrained(tl: TypeLambda, owningTree: untpd.Tree)(implicit ctx: Context): (TypeLambda, List[TypeTree]) = {
+  def constrained(tl: TypeLambda, owningTree: untpd.Tree, alwaysAddTypeVars: Boolean = false)(implicit ctx: Context): (TypeLambda, List[TypeTree]) = {
     val state = ctx.typerState
-    assert(!(ctx.typerState.isCommittable && owningTree.isEmpty),
+    val addTypeVars = alwaysAddTypeVars || !owningTree.isEmpty
+    assert(!(ctx.typerState.isCommittable && !addTypeVars),
       s"inconsistent: no typevars were added to committable constraint ${state.constraint}")
 
     def newTypeVars(tl: TypeLambda): List[TypeTree] =
@@ -392,7 +393,7 @@ object ProtoTypes {
     val added =
       if (state.constraint contains tl) tl.newLikeThis(tl.paramNames, tl.paramInfos, tl.resultType)
       else tl
-    val tvars = if (owningTree.isEmpty) Nil else newTypeVars(added)
+    val tvars = if (addTypeVars) newTypeVars(added) else Nil
     ctx.typeComparer.addToConstraint(added, tvars.tpes.asInstanceOf[List[TypeVar]])
     (added, tvars)
   }
@@ -400,13 +401,13 @@ object ProtoTypes {
   /**  Same as `constrained(tl, EmptyTree)`, but returns just the created type lambda */
   def constrained(tl: TypeLambda)(implicit ctx: Context): TypeLambda = constrained(tl, EmptyTree)._1
 
-  /** Create a new TypeParamRef that represents a dependent method parameter singleton */
-  def newDepTypeParamRef(tp: Type)(implicit ctx: Context): TypeParamRef = {
+  /** Create a new TypeVar that represents a dependent method parameter singleton */
+  def newDepTypeVar(tp: Type)(implicit ctx: Context): TypeVar = {
     val poly = PolyType(DepParamName.fresh().toTypeName :: Nil)(
         pt => TypeBounds.upper(AndType(tp, defn.SingletonType)) :: Nil,
         pt => defn.AnyType)
-    ctx.typeComparer.addToConstraint(poly, Nil)
-    TypeParamRef(poly, 0)
+    constrained(poly, untpd.EmptyTree, alwaysAddTypeVars = true)
+      ._2.head.tpe.asInstanceOf[TypeVar]
   }
 
   /** The result type of `mt`, where all references to parameters of `mt` are
@@ -415,7 +416,7 @@ object ProtoTypes {
   def resultTypeApprox(mt: MethodType)(implicit ctx: Context): Type =
     if (mt.isDependent) {
       def replacement(tp: Type) =
-        if (ctx.mode.is(Mode.TypevarsMissContext)) WildcardType else newDepTypeParamRef(tp)
+        if (ctx.mode.is(Mode.TypevarsMissContext)) WildcardType else newDepTypeVar(tp)
       mt.resultType.substParams(mt, mt.paramInfos.map(replacement))
     }
     else mt.resultType
diff --git a/tests/pos/i2152.scala b/tests/pos/i2152.scala
new file mode 100644
index 000000000..2171a487e
--- /dev/null
+++ b/tests/pos/i2152.scala
@@ -0,0 +1,7 @@
+class Contra[-D](task: AnyRef)
+object Test {
+  def narrow(task: AnyRef): Contra[task.type] = new Contra(task)
+  def ident[Before](elems: Contra[Before]): Contra[Before] = elems
+  val foo = null
+  ident(narrow(foo))
+}