Use putAttachment in exportForwarders to handle ambiguous overloads

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

Closes #21071

Author: wjoel

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

@@ -1454,7 +1454,7 @@ class Namer { typer: Typer =>
 
       addForwarders(selectors, Nil)
       val forwarders = avoidClashes(buf.toList)
-      exp.pushAttachment(ExportForwarders, forwarders)
+      exp.putAttachment(ExportForwarders, forwarders)
       forwarders
     end exportForwarders
 

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
+    }
+
+    ???
+  }
+}