Add error message for wrong number of argument patterns

compiler/src/dotty/tools/dotc/reporting/diagnostic/ErrorMessageID.java

@@ -114,6 +114,7 @@ public enum ErrorMessageID {
     TraitIsExpectedID,
     TraitRedefinedFinalMethodFromAnyRefID,
     PackageNameAlreadyDefinedID,
+    UnapplyInvalidNumberOfArgumentsID,
     ;
 
     public int errorNumber() {

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

@@ -1929,4 +1929,17 @@ object messages {
     val explanation =
       hl"An ${"object"} cannot have the same name as an existing ${"package"}. Rename either one of them."
   }
+
+  case class UnapplyInvalidNumberOfArguments(qual: untpd.Tree, argTypes: List[Type])(implicit ctx: Context)
+    extends Message(UnapplyInvalidNumberOfArgumentsID) {
+    val kind = "Syntax"
+    val msg = hl"Wrong number of argument patterns for $qual; expected: ($argTypes%, %)"
+    val explanation =
+      hl"""The Unapply method of $qual was used with incorrect number of arguments.
+          |Expected usage would be something like:
+          |case $qual(${argTypes.map(_ => '_')}%, %) => ...
+          |
+        |where subsequent arguments would have following types: ($argTypes%, %).
+        |""".stripMargin
+  }
 }

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -3,7 +3,7 @@ package dotc
 package typer
 
 import core._
-import ast.{Trees, untpd, tpd, TreeInfo}
+import ast.{TreeInfo, Trees, tpd, untpd}
 import util.Positions._
 import util.Stats.track
 import Trees.Untyped
@@ -26,13 +26,14 @@ import EtaExpansion._
 import Inferencing._
 
 import collection.mutable
-import config.Printers.{typr, unapp, overload}
+import config.Printers.{overload, typr, unapp}
 import TypeApplications._
 
 import language.implicitConversions
 import reporting.diagnostic.Message
 import reporting.trace
 import Constants.{Constant, IntTag, LongTag}
+import dotty.tools.dotc.reporting.diagnostic.messages.UnapplyInvalidNumberOfArguments
 
 import scala.collection.mutable.ListBuffer
 
@@ -969,7 +970,7 @@ trait Applications extends Compatibility { self: Typer with Dynamic =>
           case _ => args
         }
         if (argTypes.length != bunchedArgs.length) {
-          ctx.error(em"wrong number of argument patterns for $qual; expected: ($argTypes%, %)", tree.pos)
+          ctx.error(UnapplyInvalidNumberOfArguments(qual, argTypes), tree.pos)
           argTypes = argTypes.take(args.length) ++
             List.fill(argTypes.length - args.length)(WildcardType)
         }

compiler/test/dotty/tools/dotc/reporting/ErrorMessagesTests.scala

@@ -1189,7 +1189,6 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       assertEquals("method wait", method.show)
     }
 
-
   @Test def packageNameAlreadyDefined =
     checkMessagesAfter("frontend") {
       """
@@ -1203,4 +1202,26 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       val PackageNameAlreadyDefined(pkg) = messages.head
       assertEquals(pkg.show, "object bar")
     }
+
+  @Test def unapplyInvalidNumberOfArguments =
+    checkMessagesAfter("frontend") {
+      """
+        |case class Boo(a: Int, b: String)
+        |
+        |object autoTuplingNeg2 {
+        |  val z = Boo(1, "foo")
+        |
+        |  z match {
+        |    case Boo(a, b, c) => a
+        |  }
+        |}
+      """.stripMargin
+    }
+      .expect { (ictx, messages) =>
+        implicit val ctx: Context = ictx
+        assertMessageCount(1, messages)
+        val UnapplyInvalidNumberOfArguments(qual, argTypes) :: Nil = messages
+        assertEquals("Boo", qual.show)
+        assertEquals("(class Int, class String)", argTypes.map(_.typeSymbol).mkString("(", ", ", ")"))
+      }
 }