Massage the message

Author: som-snytt

Diff for: compiler/src/dotty/tools/dotc/reporting/messages.scala

@@ -2520,8 +2520,10 @@ class ExtensionHasDefault(method: Symbol)(using Context)
   def msg(using Context) =
     i"""Extension method ${hl(method.name.toString)} should not have a default argument for its receiver."""
   def explain(using Context) =
-    i"""Although extensions are ordinary methods, they must be invoked as a selection.
-       |Therefore, the receiver cannot be omitted. A default argument for that parameter would never be used."""
+    i"""Although extensions are ordinary methods, when they are invoked as a selection,
+       |the receiver cannot be omitted. A default argument for that parameter would never be used.
+       |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."""
 
 class TraitCompanionWithMutableStatic()(using Context)
   extends SyntaxMsg(TraitCompanionWithMutableStaticID) {

Diff for: tests/warn/i12460.check

@@ -5,8 +5,10 @@
   |---------------------------------------------------------------------------------------------------------------------
   | Explanation (enabled by `-explain`)
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-  | Although extensions are ordinary methods, they must be invoked as a selection.
-  | Therefore, the receiver cannot be omitted. A default argument for that parameter would never be used.
+  | Although extensions are ordinary methods, when they are invoked as a selection,
+  | the receiver cannot be omitted. A default argument for that parameter would never be used.
+  | 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.
    ---------------------------------------------------------------------------------------------------------------------
 -- [E208] Potential Issue Warning: tests/warn/i12460.scala:5:37 --------------------------------------------------------
 5 |extension (using String)(s: String = "hello, world") def revert = s.reverse.toUpperCase // warn
@@ -15,6 +17,8 @@
   |---------------------------------------------------------------------------------------------------------------------
   | Explanation (enabled by `-explain`)
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
-  | Although extensions are ordinary methods, they must be invoked as a selection.
-  | Therefore, the receiver cannot be omitted. A default argument for that parameter would never be used.
+  | Although extensions are ordinary methods, when they are invoked as a selection,
+  | the receiver cannot be omitted. A default argument for that parameter would never be used.
+  | 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.
    ---------------------------------------------------------------------------------------------------------------------