Generalize eta reduction to partially applied types.

Now also allows to reduce something like

  ([T] => Map[T, String])

to

  Map[_, String]

2 files changed, 13 insertions, 2 deletions

diff --git a/src/dotty/tools/dotc/core/TypeApplications.scala b/src/dotty/tools/dotc/core/TypeApplications.scala
index 062867353..5d856495b 100644
--- a/src/dotty/tools/dotc/core/TypeApplications.scala
+++ b/src/dotty/tools/dotc/core/TypeApplications.scala
@@ -506,9 +506,12 @@ class TypeApplications(val self: Type) extends AnyVal {
             tp.refinedInfo match {
               case TypeAlias(TypeRef(RefinedThis(rt), rname))
               if (rname == tparam.name) && (rt eq self) =>
+                // todo: add bounds
                 etaCore(tp.parent, otherParams)
               case _ =>
-                NoType
+                val pcore = etaCore(tp.parent, tparams)
+                if (pcore.exists) tp.derivedRefinedType(pcore, tp.refinedName, tp.refinedInfo)
+                else NoType
             }
           case _ =>
             NoType
@@ -518,7 +521,8 @@ class TypeApplications(val self: Type) extends AnyVal {
       case self @ RefinedType(parent, tpnme.hkApply) =>
         val lc = parent.LambdaClass(forcing = false)
         self.refinedInfo match {
-          case TypeAlias(alias) if lc.exists => etaCore(alias, lc.typeParams.reverse)
+          case TypeAlias(alias) if lc.exists =>
+            etaCore(alias, lc.typeParams.reverse)
           case _ => NoType
         }
       case _ => NoType
diff --git a/src/dotty/tools/dotc/core/TypeComparer.scala b/src/dotty/tools/dotc/core/TypeComparer.scala
index eb1dcacdd..c9b1e8794 100644
--- a/src/dotty/tools/dotc/core/TypeComparer.scala
+++ b/src/dotty/tools/dotc/core/TypeComparer.scala
@@ -1232,5 +1232,12 @@ class ExplainingTypeComparer(initctx: Context) extends TypeComparer(initctx) {
 
   override def copyIn(ctx: Context) = new ExplainingTypeComparer(ctx)
 
+  override def compareHK(projection: NamedType, other: Type, inOrder: Boolean) =
+    if (projection.name == tpnme.hkApply)
+      traceIndented(i"compareHK $projection, $other, $inOrder") {
+        super.compareHK(projection, other, inOrder)
+      }
+    else super.compareHK(projection, other, inOrder)
+
   override def toString = "Subtype trace:" + { try b.toString finally b.clear() }
 }