Fix #9155: Propagate @Alpha on export forwarders

Author: nicolasstucki

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

@@ -1129,6 +1129,9 @@ class Namer { typer: Typer =>
                 ctx.newSymbol(cls, forwarderName, mbrFlags, mbrInfo, coord = span)
               }
             forwarder.info = avoidPrivateLeaks(forwarder)
+            for annot <- sym.annotations do
+              if annot.tree.symbol.maybeOwner == defn.AlphaAnnot then
+                forwarder.addAnnotation(annot)
             val forwarderDef =
               if (forwarder.isType) tpd.TypeDef(forwarder.asType)
               else {

tests/run/i9155.scala

@@ -0,0 +1,9 @@
+object Foo:
+  @scala.annotation.alpha("w") def \/\/ = "W"
+
+object Bar:
+  export Foo._
+
+@main def Test =
+  assert(Foo.getClass.getMethods.exists(_.getName == "w"))
+  assert(Bar.getClass.getMethods.exists(_.getName == "w"))