fix #2502: improve warning message of exhaustivity check

Author: liufengyun

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

@@ -789,7 +789,12 @@ object messages {
            |It would fail on: $uncovered"""
 
 
-    val explanation = ""
+    val explanation =
+      hl"""|There are several ways to make the match exhaustive:
+           | - add missing cases as shown in the warning
+           | - change the return type of irrefutable extractors as `Some[T]` if exist
+           | - add a wildcard `_` case at the end
+           |"""
   }
 
   case class MatchCaseUnreachable()(implicit ctx: Context)

compiler/src/dotty/tools/dotc/transform/patmat/Space.scala

@@ -657,7 +657,7 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
         else if (tp.classSymbol.is(CaseClass))
         // use constructor syntax for case class
           showType(tp) + signature(tp).map(_ => "_").mkString("(", ", ", ")")
-        else if (signature(tp).nonEmpty)
+        else if (sym.is(Flags.Case) && signature(tp).nonEmpty)
           tp.classSymbol.name + signature(tp).map(_ => "_").mkString("(", ", ", ")")
         else if (decomposed) "_: " + showType(tp)
         else "_"

tests/patmat/i2502.check

@@ -0,0 +1 @@
+5: Pattern Match Exhaustivity: _: BTypes.ClassBType

tests/patmat/i2502.scala

@@ -0,0 +1,17 @@
+abstract class BTypes {
+  trait BType
+
+  sealed trait RefBType extends BType {
+    def classOrArrayType: String = this match {
+      case ClassBType(internalName) => internalName
+      case a: ArrayBType            => ""
+    }
+  }
+
+  final class ClassBType(val internalName: String) extends RefBType
+  class ArrayBType extends RefBType
+
+  object ClassBType {
+    def unapply(x: ClassBType): Option[String] = None
+  }
+}

tests/patmat/t4691.check

@@ -1 +1 @@
-15: Pattern Match Exhaustivity: NodeType2(_)
+15: Pattern Match Exhaustivity: _: NodeType2