Tweak ExtensionNullifiedByMember

compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2504,12 +2504,15 @@ class ExtensionNullifiedByMember(method: Symbol, target: Symbol)(using Context)
   extends Message(ExtensionNullifiedByMemberID):
   def kind = MessageKind.PotentialIssue
   def msg(using Context) =
-    i"""Extension method ${hl(method.name.toString)} will never be selected
-       |because ${hl(target.name.toString)} already has a member with the same name and compatible parameter types."""
+    val targetName = hl(target.name.toString)
+    i"""Extension method ${hl(method.name.toString)} will never be selected from type $targetName
+       |because $targetName already has a member with the same name and compatible parameter types."""
   def explain(using Context) =
-    i"""An extension method can be invoked as a regular method, but if that is intended,
+    i"""Although extensions can be overloaded, they do not overload existing member methods.
+       |An extension method can be invoked as a regular method, but if that is the intended usage,
        |it should not be defined as an extension.
-       |Although extensions can be overloaded, they do not overload existing member methods."""
+       |
+       |The extension may be invoked as though selected from an arbitrary type if conversions are in play."""
 
 class TraitCompanionWithMutableStatic()(using Context)
   extends SyntaxMsg(TraitCompanionWithMutableStaticID) {

tests/warn/i22267.check

@@ -0,0 +1,14 @@
+-- [E194] Potential Issue Warning: tests/warn/i22267.scala:13:26 -------------------------------------------------------
+13 |  extension (self: C) def m(n: Double): Unit = println(2->n) // warn
+   |                          ^
+   |                          Extension method m will never be selected from type C
+   |                          because C already has a member with the same name and compatible parameter types.
+   |--------------------------------------------------------------------------------------------------------------------
+   | Explanation (enabled by `-explain`)
+   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+   | Although extensions can be overloaded, they do not overload existing member methods.
+   | An extension method can be invoked as a regular method, but if that is the intended usage,
+   | it should not be defined as an extension.
+   |
+   | The extension may be invoked as though selected from an arbitrary type if conversions are in play.
+    --------------------------------------------------------------------------------------------------------------------

tests/warn/i22267.scala

@@ -0,0 +1,28 @@
+//> using options -explain
+
+import language.implicitConversions
+
+case class M[T](value: T)
+given [T]: Conversion[M[T], T] = _.value
+class C:
+  def m(n: Double): Unit = println(0->n)
+object C:
+  given Ops = Ops()
+class Ops:
+  extension (self: C) def m(n: Int): Unit = println(1->n)
+  extension (self: C) def m(n: Double): Unit = println(2->n) // warn
+  extension (self: C) def m(s: String): Unit = println(3->s)
+
+@main def test() =
+  val c = M(C())
+  def i = 42
+  def pi = 3.14
+  c.value.m(i)
+  c.value.m(pi)
+  c.m(i) // conversion
+  c.m(pi) // conversion
+  c.m("hello, world") // extension
+  //m(c)(pi)
+  val c0 = C()
+  c0.m(pi)
+  c0.m("hello, world")