Merge pull request #11926 from dotty-staging/fix-11922

Eta expand exported type constructors

Author: smarter

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

@@ -1032,10 +1032,13 @@ class Namer { typer: Typer =>
             val forwarder =
               if mbr.isType then
                 val forwarderName = checkNoConflict(alias.toTypeName, isPrivate = false, span)
+                var target = path.tpe.select(sym)
+                if target.typeParams.nonEmpty then
+                  target = target.EtaExpand(target.typeParams)
                 newSymbol(
                   cls, forwarderName,
                   Exported | Final,
-                  TypeAlias(path.tpe.select(sym)),
+                  TypeAlias(target),
                   coord = span)
                 // Note: This will always create unparameterzied aliases. So even if the original type is
                 // a parameterized class, say `C[X]` the alias will read `type C = d.C`. We currently do

tests/pos/i11922.scala

@@ -0,0 +1,17 @@
+object example {
+  trait MyType[A]
+  type Alias[A, B] = MyType[B]
+}
+
+object bug {
+  export example.{MyType, Alias}
+  def bug[A](m: MyType[A]): MyType[A] = m
+  val bug2: MyType[String] => MyType[String] = m => m
+  def bug3[A, B](m: Alias[A, B]): MyType[B] = m
+  def bug4[A, B](m: Alias[A, B]): Alias[Int, B] = m
+
+  //it works when referencing the original type in the parameter position.
+  def thisWorks[A](m: example.MyType[A]): MyType[A] = m
+  val thisWorks2: example.MyType[String] => MyType[String] = m => m
+  val thisWorks3: MyType[String] = (??? : MyType[String])
+}