Convert "method has return statement; needs result type" to new error format with MissingReturnTypeWithReturnStatement

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

@@ -96,6 +96,7 @@ public enum ErrorMessageID {
     WrongNumberOfParametersID,
     DuplicatePrivateProtectedQualifierID,
     ExpectedStartOfTopLevelDefinitionID,
+    MissingReturnTypeWithReturnStatementID,
     ;
 
     public int errorNumber() {

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

@@ -598,6 +598,21 @@ object messages {
            |}"""
   }
 
+  case class MissingReturnTypeWithReturnStatement(method: Symbol)(implicit ctx: Context)
+  extends Message(MissingReturnTypeWithReturnStatementID) {
+    val kind = "Syntax"
+    val msg = hl"$method has a return statement; it needs a result type"
+    val explanation =
+      hl"""|If a method contains a ${"return"} statement, it must have a return
+           |type explicitly given to it. For example:
+           |
+           |${"def bad() = { return \"fail\" }"}
+           |
+           |This is illegal because bad does not have a return type given
+           |to it. Giving bad a return type of ${"String"} will resolve
+           |this error."""
+  }
+
   case class YieldOrDoExpectedInForComprehension()(implicit ctx: Context)
   extends Message(YieldOrDoExpectedInForComprehensionID) {
     val kind = "Syntax"

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

@@ -987,7 +987,7 @@ class Typer extends Namer with TypeAssigner with Applications with Implicits wit
         if (owner.isInlineMethod)
           (EmptyTree, errorType(em"no explicit return allowed from inline $owner", tree.pos))
         else if (!owner.isCompleted)
-          (EmptyTree, errorType(em"$owner has return statement; needs result type", tree.pos))
+          (EmptyTree, errorType(MissingReturnTypeWithReturnStatement(owner), tree.pos))
         else {
           val from = Ident(TermRef(NoPrefix, owner.asTerm))
           val proto = returnProto(owner, cx.scope)

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

@@ -955,4 +955,21 @@ class ErrorMessagesTests extends ErrorMessagesTest {
 
         assertEquals(ExpectedStartOfTopLevelDefinition(), err)
       }
+
+  @Test def missingReturnTypeWithReturnStatement =
+    checkMessagesAfter("frontend") {
+      """class BadFunction {
+        |  def bad() = { return "fail" }
+        |}
+      """.stripMargin
+    }.expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+      val defn = ictx.definitions
+
+      assertMessageCount(1, messages)
+
+      val MissingReturnTypeWithReturnStatement(method) :: Nil = messages
+
+      assertEquals(method.name.toString, "bad")
+    }
 }