Add an error message for "invalid unapply return type" error.

Part of #1589.

Author: Marc Karassev

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

@@ -112,6 +112,7 @@ public enum ErrorMessageID {
     UndefinedNamedTypeParameterID,
     IllegalStartOfStatementID,
     TraitIsExpectedID,
+    InvalidUnapplyReturnTypeID,
     ;
 
     public int errorNumber() {

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

@@ -1916,4 +1916,32 @@ object messages {
           |"""
     }
   }
+
+  case class InvalidUnapplyReturnType(unapplyResult: Type, unapplyName: Symbol#ThisName)(implicit ctx: Context)
+    extends Message(InvalidUnapplyReturnTypeID) {
+    val kind = "Type Mismatch"
+    val msg = hl"""| ${Red(i"$unapplyResult")} is not a valid result type of an $unapplyName method of an ${Magenta("extractor")}."""
+    val explanation =
+      hl"""
+          |To be used as an ${Magenta("extractor")}, an unapply method has to return an ${Green("Option[T]")} or a ${Green("Boolean")}:
+          |
+          |class A(val i: Int)
+          |
+          |object B {
+          |  def unapply(a: A): ${Green("Option[Int]")} = Some(a.i)
+          |}
+          |
+          |object C {
+          |  def unapply(a: A): ${Green("Boolean")} = a.i == 2
+          |}
+          |
+          |object Test {
+          |  def test(a: A) = a match {
+          |    ${Magenta("case B(1)")} => 1
+          |    ${Magenta("case a @ C()")} => 2
+          |  }
+          |}
+        """.stripMargin
+  }
+
 }

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

@@ -33,6 +33,7 @@ import language.implicitConversions
 import reporting.diagnostic.Message
 import reporting.trace
 import Constants.{Constant, IntTag, LongTag}
+import dotty.tools.dotc.reporting.diagnostic.messages.InvalidUnapplyReturnType
 
 import scala.collection.mutable.ListBuffer
 
@@ -92,7 +93,7 @@ object Applications {
     def getTp = extractorMemberType(unapplyResult, nme.get, pos)
 
     def fail = {
-      ctx.error(i"$unapplyResult is not a valid result type of an $unapplyName method of an extractor", pos)
+      ctx.error(InvalidUnapplyReturnType(unapplyResult, unapplyName))
       Nil
     }
 

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

@@ -1172,4 +1172,25 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       val TraitIsExpected(symbol) :: Nil = messages
       assertEquals("class B", symbol.show)
     }
+
+  @Test def invalidUnapplyReturnType =
+    checkMessagesAfter("frontend") {
+      """
+        |class A(val i: Int)
+        |
+        |object A {
+        |  def unapply(a: A): Int = a.i
+        |  def test(a: A) = a match {
+        |    case A() => 1
+        |  }
+        |}
+      """.stripMargin
+    }.expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+      assertMessageCount(1, messages)
+      val InvalidUnapplyReturnType(unapplyResult, unapplyName) :: Nil = messages
+      assertEquals("Int", unapplyResult.show)
+      assertEquals("unapply", unapplyName.show)
+    }
+
 }