Fix #2502: improve warning message of exhaustivity check

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
+           | - If an extractor always return 'Some(...)', write 'Some[X]' for its return type
+           | - Add a 'case _ => ...' at the end to match all remaining cases
+           |"""
   }
 
   case class MatchCaseUnreachable()(implicit ctx: Context)

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

@@ -641,6 +641,14 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
 
   /** Display spaces */
   def show(s: Space): String = {
+
+    /** does the companion object of the given symbol have custom unapply */
+    def hasCustomUnapply(sym: Symbol): Boolean = {
+      val companion = sym.companionModule
+      companion.findMember(nme.unapply, NoPrefix, excluded = Synthetic).exists ||
+        companion.findMember(nme.unapplySeq, NoPrefix, excluded = Synthetic).exists
+    }
+
     def doShow(s: Space, mergeList: Boolean = false): String = s match {
       case Empty => ""
       case Typ(c: ConstantType, _) => c.value.show
@@ -654,11 +662,9 @@ class SpaceEngine(implicit ctx: Context) extends SpaceLogic {
           if (mergeList) "_*" else "_: List"
         else if (scalaConsType.isRef(sym))
           if (mergeList) "_" else "List(_)"
-        else if (tp.classSymbol.is(CaseClass))
+        else if (tp.classSymbol.is(CaseClass) && !hasCustomUnapply(tp.classSymbol))
         // use constructor syntax for case class
           showType(tp) + signature(tp).map(_ => "_").mkString("(", ", ", ")")
-        else if (signature(tp).nonEmpty)
-          tp.classSymbol.name + signature(tp).map(_ => "_").mkString("(", ", ", ")")
         else if (decomposed) "_: " + showType(tp)
         else "_"
       case Kon(tp, params) =>

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/i2502b.check

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

tests/patmat/i2502b.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 case class ClassBType(val internalName: String) extends RefBType
+  class ArrayBType extends RefBType
+
+  object ClassBType {
+    def unapply(x: RefBType): Option[String] = None
+  }
+}

tests/patmat/t4691.check

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