Re-use attachment in exportForwarders to handle ambiguous overloads

exportForwarders can be called more than once for the same expression
if there are ambiguous overloads.

Just return the already computed ExportForwarders if that happens.

Closes #21071

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

@@ -1452,10 +1452,11 @@ class Namer { typer: Typer =>
           forwarders.derivedCons(forwarder2, avoidClashes(forwarders2))
         case Nil => forwarders
 
-      addForwarders(selectors, Nil)
-      val forwarders = avoidClashes(buf.toList)
-      exp.pushAttachment(ExportForwarders, forwarders)
-      forwarders
+      exp.getAttachment(ExportForwarders).getOrElse:
+        addForwarders(selectors, Nil)
+        val forwarders = avoidClashes(buf.toList)
+        exp.pushAttachment(ExportForwarders, forwarders)
+        forwarders
     end exportForwarders
 
     /** Add forwarders as required by the export statements in this class */

tests/neg/i21071.check

@@ -0,0 +1,9 @@
+-- [E051] Reference Error: tests/neg/i21071.scala:9:2 ------------------------------------------------------------------
+9 |  foo { // error
+  |  ^^^
+  |  Ambiguous overload. The overloaded alternatives of method foo in object MySuite with types
+  |   (a: String): Nothing
+  |   (a: List[String]): Nothing
+  |  both match arguments ((??? : => Nothing))
+  |
+  | longer explanation available when compiling with `-explain`

tests/neg/i21071.scala

@@ -0,0 +1,21 @@
+trait Service {
+  def method: String
+}
+
+object MySuite {
+  def foo(a: List[String]) = ???
+  def foo(a: String) = ???
+
+  foo { // error
+
+    new Service {
+      private val underlying: Service = ???
+      private val s = "foo"
+
+      export underlying.*
+      export s.toLowerCase
+    }
+
+    ???
+  }
+}