Add example to explanation

[Cherry-picked eed38ec]

Author: odersky

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

@@ -2357,8 +2357,18 @@ class ExtractorNotFound(val name: Name)(using Context) extends NotFoundMsg(Extra
   def msg(using Context) = i"no pattern match extractor named $name was found"
   def explain(using Context) =
     i"""An application $name(...) in a pattern can refer to an extractor
-       |which defines an unapply or unapplySeq method. Case classes and enum cases
-       |implicitly define extractors with the name of the class or enum case.
+       |which defines an unapply or unapplySeq method. Example:
+       |
+       |  object split:
+       |    def unapply(x: String) =
+       |      val (leading, trailing) = x.splitAt(x.length / 2)
+       |      Some((leading, trailing))
+       |
+       |  val split(fst, snd) = "HiHo"
+       |
+       |The extractor pattern `split(fst, snd)` defines `fst` as the first half "Hi" and
+       |`snd` as the second half "Ho" of the right hand side "HiHo". Case classes and
+       |enum cases implicitly define extractors with the name of the class or enum case.
        |Here, no extractor named $name was found, so the pattern could not be typed."""
 
 class MemberWithSameNameAsStatic()(using Context)

tests/neg/i18684.check

@@ -6,8 +6,18 @@
   | Explanation (enabled by `-explain`)
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   | An application s(...) in a pattern can refer to an extractor
-  | which defines an unapply or unapplySeq method. Case classes and enum cases
-  | implicitly define extractors with the name of the class or enum case.
+  | which defines an unapply or unapplySeq method. Example:
+  |
+  |   object split:
+  |     def unapply(x: String) =
+  |       val (leading, trailing) = x.splitAt(x.length / 2)
+  |       Some((leading, trailing))
+  |
+  |   val split(fst, snd) = "HiHo"
+  |
+  | The extractor pattern `split(fst, snd)` defines `fst` as the first half "Hi" and
+  | `snd` as the second half "Ho" of the right hand side "HiHo". Case classes and
+  | enum cases implicitly define extractors with the name of the class or enum case.
   | Here, no extractor named s was found, so the pattern could not be typed.
    ---------------------------------------------------------------------------------------------------------------------
 -- [E189] Not Found Error: tests/neg/i18684.scala:5:6 ------------------------------------------------------------------
@@ -18,8 +28,18 @@
   | Explanation (enabled by `-explain`)
   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
   | An application i(...) in a pattern can refer to an extractor
-  | which defines an unapply or unapplySeq method. Case classes and enum cases
-  | implicitly define extractors with the name of the class or enum case.
+  | which defines an unapply or unapplySeq method. Example:
+  |
+  |   object split:
+  |     def unapply(x: String) =
+  |       val (leading, trailing) = x.splitAt(x.length / 2)
+  |       Some((leading, trailing))
+  |
+  |   val split(fst, snd) = "HiHo"
+  |
+  | The extractor pattern `split(fst, snd)` defines `fst` as the first half "Hi" and
+  | `snd` as the second half "Ho" of the right hand side "HiHo". Case classes and
+  | enum cases implicitly define extractors with the name of the class or enum case.
   | Here, no extractor named i was found, so the pattern could not be typed.
    ---------------------------------------------------------------------------------------------------------------------
 -- [E189] Not Found Error: tests/neg/i18684.scala:10:8 -----------------------------------------------------------------
@@ -30,8 +50,18 @@
    | Explanation (enabled by `-explain`)
    |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
    | An application foo(...) in a pattern can refer to an extractor
-   | which defines an unapply or unapplySeq method. Case classes and enum cases
-   | implicitly define extractors with the name of the class or enum case.
+   | which defines an unapply or unapplySeq method. Example:
+   |
+   |   object split:
+   |     def unapply(x: String) =
+   |       val (leading, trailing) = x.splitAt(x.length / 2)
+   |       Some((leading, trailing))
+   |
+   |   val split(fst, snd) = "HiHo"
+   |
+   | The extractor pattern `split(fst, snd)` defines `fst` as the first half "Hi" and
+   | `snd` as the second half "Ho" of the right hand side "HiHo". Case classes and
+   | enum cases implicitly define extractors with the name of the class or enum case.
    | Here, no extractor named foo was found, so the pattern could not be typed.
     --------------------------------------------------------------------------------------------------------------------
 -- [E127] Pattern Match Error: tests/neg/i18684.scala:12:6 -------------------------------------------------------------