Update error message at Parsers.scala:1901.

This one is about a 'missing return type' when we can't infer a type. I used tests/ng/i871.scala to verify the change

Dropped a closing brace on a rebase

Author: ljdelight

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

@@ -1898,7 +1898,7 @@ object Parsers {
             else EmptyTree
           }
           else {
-            if (!isExprIntro) syntaxError("missing return type", in.lastOffset)
+            if (!isExprIntro) syntaxError(MissingReturnType(), in.lastOffset)
             accept(EQUALS)
             expr()
           }

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

@@ -275,10 +275,10 @@ object messages {
 
     val explanation = ""
   }
- 
+
   case class EarlyDefinitionsNotSupported()(implicit ctx:Context) extends Message(9) {
     val kind = "Syntax"
-   
+
     val msg = "early definitions are not supported; use trait parameters instead"
 
     val code1 =
@@ -482,4 +482,15 @@ object messages {
         |but an uninitialized var definition
         """.stripMargin
   }
+
+  case class MissingReturnType()(implicit ctx:Context) extends Message(17) {
+    val kind = "Syntax"
+    val msg = "missing return type"
+    val explanation =
+      hl"""The return type is missing and needs to be clarified. For example, `area` returns a ${"Double"}:
+        |
+        |trait Shape {
+        |  def area: Double
+        |}""".stripMargin
+  }
 }