Add error messages - Parsers.scala:712 (#1842)

* Add error messages - Parsers.scala:712
* Tidy up imports and formatting

Author: raindeerr

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -730,7 +730,7 @@ object Parsers {
 
       in.token match {
         case ARROW => functionRest(t :: Nil)
-        case FORSOME => syntaxError("existential types no longer supported; use a wildcard type or dependent type instead"); t
+        case FORSOME => syntaxError(ExistentialTypesNoLongerSupported()); t
         case _ => t
       }
     }

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

@@ -5,8 +5,7 @@ package diagnostic
 
 import dotc.core._
 import Contexts.Context, Decorators._, Symbols._, Names._, NameOps._, Types._
-import util.{SourceFile, NoSource}
-import util.{SourcePosition, NoSourcePosition}
+import util.SourcePosition
 import config.Settings.Setting
 import interfaces.Diagnostic.{ERROR, WARNING, INFO}
 import printing.Highlighting._
@@ -901,4 +900,27 @@ object messages {
     val msg = hl"trying to define package with same name as `$existing`"
     val explanation = ""
   }
+
+  case class ExistentialTypesNoLongerSupported()(implicit ctx: Context) extends Message(34) {
+    val kind = "Syntax"
+    val msg =
+      hl"""|Existential types are no longer supported -
+           |use a wildcard or dependent type instead"""
+    val explanation =
+      hl"""|The use of existential types is no longer supported.
+           |
+           |You should use a wildcard or dependent type instead.
+           |
+           |For example:
+           |
+           |Instead of using ${"forSome"} to specify a type variable
+           |
+           |${"List[T forSome { type T }]"}
+           |
+           |Try using a wildcard type variable
+           |
+           |${"List[_]"}
+           |"""
+  }
+
 }