Turn on -explain in test

[Cherry-picked 8df6b48]

Author: odersky

tests/neg/i18684.check

@@ -2,23 +2,51 @@
 3 |  val s(): String = "hello, world" // error
   |      ^
   |      no pattern match extractor named s was found
-  |
-  | longer explanation available when compiling with `-explain`
+  |---------------------------------------------------------------------------------------------------------------------
+  | 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.
+  | Here, no extractor named s was found, so the pattern could not be typed.
+   ---------------------------------------------------------------------------------------------------------------------
 -- [E189] Not Found Error: tests/neg/i18684.scala:5:6 ------------------------------------------------------------------
 5 |  val i() = 22 // error
   |      ^
   |      no pattern match extractor named i was found
-  |
-  | longer explanation available when compiling with `-explain`
+  |---------------------------------------------------------------------------------------------------------------------
+  | 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.
+  | Here, no extractor named i was found, so the pattern could not be typed.
+   ---------------------------------------------------------------------------------------------------------------------
 -- [E189] Not Found Error: tests/neg/i18684.scala:10:8 -----------------------------------------------------------------
 10 |    val foo() = "33" // error
    |        ^^^
    |        no pattern match extractor named foo was found
-   |
-   | longer explanation available when compiling with `-explain`
+   |--------------------------------------------------------------------------------------------------------------------
+   | 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.
+   | Here, no extractor named foo was found, so the pattern could not be typed.
+    --------------------------------------------------------------------------------------------------------------------
 -- [E127] Pattern Match Error: tests/neg/i18684.scala:12:6 -------------------------------------------------------------
 12 |  val inner(x) = 3 // error
    |      ^^^^^
    |      Test.inner cannot be used as an extractor in a pattern because it lacks an unapply or unapplySeq method
+   |--------------------------------------------------------------------------------------------------------------------
+   | Explanation (enabled by `-explain`)
+   |- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+   | An unapply method should be defined in an object as follow:
+   |   - If it is just a test, return a Boolean. For example case even()
+   |   - If it returns a single sub-value of type T, return an Option[T]
+   |   - If it returns several sub-values T1,...,Tn, group them in an optional tuple Option[(T1,...,Tn)]
    |
-   | longer explanation available when compiling with `-explain`
+   | Sometimes, the number of sub-values isn't fixed and we would like to return a sequence.
+   | For this reason, you can also define patterns through unapplySeq which returns Option[Seq[T]].
+   | This mechanism is used for instance in pattern case List(x1, ..., xn)
+    --------------------------------------------------------------------------------------------------------------------

tests/neg/i18684.scala

@@ -1,4 +1,4 @@
-//> using option -explain
+//> using options -explain
 object Test:
   val s(): String = "hello, world" // error