Add explanation to "already defined" message

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) {

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

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
+