Fix Java generic signatures for aliases

Author: smarter

compiler/src/dotty/tools/dotc/transform/GenericSignatures.scala

@@ -455,7 +455,7 @@ object GenericSignatures {
   private class NeedsSigCollector(using Context) extends TypeAccumulator[Boolean] {
     override def apply(x: Boolean, tp: Type): Boolean =
       if (!x)
-        tp match {
+        tp.dealias match {
           case RefinedType(parent, refinedName, refinedInfo) =>
             val sym = parent.typeSymbol
             if (sym == defn.ArrayClass) foldOver(x, refinedInfo)
@@ -471,9 +471,7 @@ object GenericSignatures {
             foldOver(tp.typeParams.nonEmpty, parents)
           case AnnotatedType(tpe, _) =>
             foldOver(x, tpe)
-          case proxy: TypeProxy =>
-            foldOver(x, proxy)
-          case _ =>
+          case tp =>
             foldOver(x, tp)
         }
       else x

tests/generic-java-signatures/aliases.check

@@ -0,0 +1 @@
+scala.collection.immutable.List<java.lang.String>

tests/generic-java-signatures/aliases.scala

@@ -0,0 +1,10 @@
+class Foo {
+  type A = List[String]
+  def foo(): A = Nil
+}
+
+object Test {
+  def main(args: Array[String]): Unit = {
+    println(classOf[Foo].getDeclaredMethod("foo").getGenericReturnType)
+  }
+}