From d6453ec28ad2fa3bc332acb80ddb657137f2759d Mon Sep 17 00:00:00 2001
From: Martin Odersky <odersky@gmail.com>
Date: Fri, 16 Jul 2021 09:59:53 +0200
Subject: [PATCH] Add explanation to "already defined" message

Fixes #13089
---
 .../dotty/tools/dotc/reporting/messages.scala |  6 +++++-
 tests/neg/i13089.check                        | 12 +++++++++++
 tests/neg/i13089.scala                        | 21 +++++++++++++++++++
 3 files changed, 38 insertions(+), 1 deletion(-)
 create mode 100644 tests/neg/i13089.check
 create mode 100644 tests/neg/i13089.scala

diff --git a/compiler/src/dotty/tools/dotc/reporting/messages.scala b/compiler/src/dotty/tools/dotc/reporting/messages.scala
index fb7c6d41e885..0e47c4a1ff57 100644
--- a/compiler/src/dotty/tools/dotc/reporting/messages.scala
+++ b/compiler/src/dotty/tools/dotc/reporting/messages.scala
@@ -1861,11 +1861,15 @@ import transform.SymUtils._
         i" in ${conflicting.associatedFile}"
       else if conflicting.owner == owner then ""
       else i" in ${conflicting.owner}"
+    private def note =
+      if owner.is(Method) || conflicting.is(Method) then
+        "\n\nNote that overloaded methods must all be defined in the same group of toplevel definitions"
+      else ""
     def msg =
       if conflicting.isTerm != name.isTermName then
         em"$name clashes with $conflicting$where; the two must be defined together"
       else
-        em"$name is already defined as $conflicting$where"
+        em"$name is already defined as $conflicting$where$note"
     def explain = ""
 
   class PackageNameAlreadyDefined(pkg: Symbol)(using Context) extends NamingMsg(PackageNameAlreadyDefinedID) {
diff --git a/tests/neg/i13089.check b/tests/neg/i13089.check
new file mode 100644
index 000000000000..ed0f9d5effd9
--- /dev/null
+++ b/tests/neg/i13089.check
@@ -0,0 +1,12 @@
+-- [E161] Naming Error: tests/neg/i13089.scala:6:8 ---------------------------------------------------------------------
+6 |    def fails : Unit = {} // error
+  |    ^^^^^^^^^^^^^^^^^^^^^
+  |    fails is already defined as method fails in tests/neg/i13089.scala
+  |
+  |    Note that overloaded methods must all be defined in the same group of toplevel definitions
+-- [E161] Naming Error: tests/neg/i13089.scala:8:6 ---------------------------------------------------------------------
+8 |  def baz(x: String): Boolean = true // error
+  |  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
+  |  baz is already defined as method baz in tests/neg/i13089.scala
+  |
+  |  Note that overloaded methods must all be defined in the same group of toplevel definitions
diff --git a/tests/neg/i13089.scala b/tests/neg/i13089.scala
new file mode 100644
index 000000000000..5b91996de837
--- /dev/null
+++ b/tests/neg/i13089.scala
@@ -0,0 +1,21 @@
+package pkg:
+
+  trait Bar
+
+  extension (bar : Bar)
+    def fails : Unit = {} // error
+
+  def baz(x: String): Boolean = true // error
+
+package pkg:
+
+  trait Foo
+  extension (foo : Foo)
+    def fails : Unit = {}
+    def works : Unit = {}
+
+  extension (bar : Bar)
+    def works : Unit = {}
+
+  def baz(x: Int): Boolean = true
+