Fix #4127: Fix mods string of FunctionTypeNeedsNonEmptyParameterList

Author: nicolasstucki

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

@@ -1715,10 +1715,11 @@ object messages {
     }
   }
 
-  case class FunctionTypeNeedsNonEmptyParameterList(isImplicit: Boolean = true, isErased: Boolean = true)(implicit ctx: Context)
+  case class FunctionTypeNeedsNonEmptyParameterList(isImplicit: Boolean, isErased: Boolean)(implicit ctx: Context)
     extends Message(FunctionTypeNeedsNonEmptyParameterListID) {
+    assert(isImplicit || isErased)
     val kind = "Syntax"
-    val mods = ((isImplicit, "implicit") :: (isErased, "erased") :: Nil).filter(_._1).mkString(" ")
+    val mods = ((isErased, "erased") :: (isImplicit, "implicit") :: Nil).collect { case (true, mod) => mod }.mkString(" ")
     val msg = mods + " function type needs non-empty parameter list"
     val explanation = {
       val code1 = s"type Transactional[T] = $mods Transaction => T"

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

@@ -764,7 +764,7 @@ class Typer extends Namer
       case tree: untpd.NonEmptyFunction =>
         if (args.nonEmpty) (tree.mods.is(Implicit), tree.mods.is(Erased))
         else {
-          ctx.error(FunctionTypeNeedsNonEmptyParameterList(), tree.pos)
+          ctx.error(FunctionTypeNeedsNonEmptyParameterList(tree.mods.is(Implicit), tree.mods.is(Erased)), tree.pos)
           (false, false)
         }
       case _ => (false, false)

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

@@ -881,7 +881,7 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       implicit val ctx: Context = ictx
 
       assertMessageCount(2, messages)
-      messages.foreach(assertEquals(_, FunctionTypeNeedsNonEmptyParameterList()))
+      messages.foreach(assertEquals(_, FunctionTypeNeedsNonEmptyParameterList(isImplicit = true, isErased = false)))
     }
 
   @Test def wrongNumberOfParameters =
@@ -1306,4 +1306,46 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       val DoubleDeclaration(symbol, previousSymbol) :: Nil = messages
       assertEquals(symbol.name.mangledString, "a")
   }
+
+  @Test def i4127a =
+    checkMessagesAfter("frontend") {
+      """
+        |class Foo {
+        |  val x: implicit () => Int = () => 1
+        |}
+      """.stripMargin
+    }.expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+      assertMessageCount(1, messages)
+      val (msg @ FunctionTypeNeedsNonEmptyParameterList(_, _)) :: Nil = messages
+      assertEquals(msg.mods, "implicit")
+    }
+
+  @Test def i4127b =
+    checkMessagesAfter("frontend") {
+      """
+        |class Foo {
+        |  val x: erased () => Int = () => 1
+        |}
+      """.stripMargin
+    }.expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+      assertMessageCount(1, messages)
+      val (msg @ FunctionTypeNeedsNonEmptyParameterList(_, _)) :: Nil = messages
+      assertEquals(msg.mods, "erased")
+    }
+
+  @Test def i4127c =
+    checkMessagesAfter("frontend") {
+      """
+        |class Foo {
+        |  val x: erased implicit () => Int = () => 1
+        |}
+      """.stripMargin
+    }.expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+      assertMessageCount(1, messages)
+      val (msg @ FunctionTypeNeedsNonEmptyParameterList(_, _)) :: Nil = messages
+      assertEquals(msg.mods, "erased implicit")
+    }
 }