From 634fcd12976bbc2877c71b8a850253791907d5d7 Mon Sep 17 00:00:00 2001
From: Joel Wilsson <joel.wilsson@gmail.com>
Date: Fri, 30 Aug 2024 14:45:51 +0200
Subject: [PATCH] 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
---
 .../src/dotty/tools/dotc/typer/Namer.scala    |  9 ++++----
 tests/neg/i21071.check                        |  9 ++++++++
 tests/neg/i21071.scala                        | 21 +++++++++++++++++++
 3 files changed, 35 insertions(+), 4 deletions(-)
 create mode 100644 tests/neg/i21071.check
 create mode 100644 tests/neg/i21071.scala

diff --git a/compiler/src/dotty/tools/dotc/typer/Namer.scala b/compiler/src/dotty/tools/dotc/typer/Namer.scala
index 0a1a70b98bbb..6167db62fbe0 100644
--- a/compiler/src/dotty/tools/dotc/typer/Namer.scala
+++ b/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 */
diff --git a/tests/neg/i21071.check b/tests/neg/i21071.check
new file mode 100644
index 000000000000..b2a3233a31c0
--- /dev/null
+++ b/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`
diff --git a/tests/neg/i21071.scala b/tests/neg/i21071.scala
new file mode 100644
index 000000000000..ac222cad7936
--- /dev/null
+++ b/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
+    }
+
+    ???
+  }
+}