Fix #7219: export forwarder should use TypeAlias for parameterless class

Author: liufengyun

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

@@ -982,7 +982,8 @@ class Namer { typer: Typer =>
              */
             def fwdInfo(ref: Type, info: Type): Type = info match {
               case _: ClassInfo =>
-                HKTypeLambda.fromParams(info.typeParams, ref)
+                if (info.typeParams.isEmpty) TypeAlias(ref)
+                else HKTypeLambda.fromParams(info.typeParams, ref)
               case _: TypeBounds =>
                 TypeAlias(ref)
               case info: HKTypeLambda =>

tests/pos-special/fatal-warnings/i7219.scala

@@ -0,0 +1,24 @@
+object Foo {
+  enum MyEnum {
+    case Red
+    case Blue(msg: String)
+  }
+  export MyEnum._
+}
+
+object Bar {
+  type Blue = Foo.Blue
+  
+  // Related Issue -- my expectation is that
+  // `export Foo.Blue` should be equivalent to
+  // `type Blue = Foo.Blue`, but it's not:
+  
+  // export Foo.Blue // Uncommenting this (and commenting `type Blue = ...`) results in compile error
+}
+
+import Foo._
+
+def foo(a: MyEnum): Seq[Bar.Blue] = a match {
+  case Red => Seq.empty
+  case m: Foo.Blue => Seq(m)
+}

tests/pos/i7219.scala

@@ -0,0 +1,9 @@
+class Foo {
+  object MyEnum {
+    class Blue
+  }
+  export MyEnum._
+
+  val a: MyEnum.Blue = ???
+  a : Blue   // ok
+}