Ref #1589: Add warning messages for migration annotation

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

@@ -118,6 +118,8 @@ public enum ErrorMessageID {
     StaticFieldsOnlyAllowedInObjectsID,
     CyclicInheritanceID,
     UnableToExtendSealedClassID,
+    SymbolHasUnparsableVersionNumberID,
+    SymbolChangedSemanticsInVersionID,
     UnableToEmitSwitchID,
     MissingCompanionForStaticID,
     ;

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

@@ -21,6 +21,7 @@ import ErrorMessageID._
 import Denotations.SingleDenotation
 import dotty.tools.dotc.ast.Trees
 import dotty.tools.dotc.ast.untpd.Modifiers
+import dotty.tools.dotc.config.ScalaVersion
 import dotty.tools.dotc.core.Flags.{FlagSet, Mutable}
 import dotty.tools.dotc.core.SymDenotations.SymDenotation
 import scala.util.control.NonFatal
@@ -140,7 +141,7 @@ object messages {
   }
 
   case class EmptyCatchBlock(tryBody: untpd.Tree)(implicit ctx: Context)
- extends EmptyCatchOrFinallyBlock(tryBody, EmptyCatchBlockID) {
+  extends EmptyCatchOrFinallyBlock(tryBody, EmptyCatchBlockID) {
     val kind = "Syntax"
     val msg =
       hl"""|The ${"catch"} block does not contain a valid expression, try
@@ -1973,6 +1974,31 @@ object messages {
     val explanation = "A sealed class or trait can only be extended in the same file as its declaration"
   }
 
+  case class SymbolHasUnparsableVersionNumber(symbol: Symbol, migrationMessage: String)(implicit ctx: Context)
+  extends Message(SymbolHasUnparsableVersionNumberID) {
+    val kind = "Syntax"
+    val msg = hl"${symbol.showLocated} has an unparsable version number: $migrationMessage"
+    val explanation =
+      hl"""$migrationMessage
+          |
+          |The ${symbol.showLocated} is marked with ${"@migration"} indicating it has changed semantics
+          |between versions and the ${"-Xmigration"} settings is used to warn about constructs
+          |whose behavior may have changed since version change."""
+  }
+
+  case class SymbolChangedSemanticsInVersion(
+    symbol: Symbol,
+    migrationVersion: ScalaVersion
+  )(implicit ctx: Context) extends Message(SymbolChangedSemanticsInVersionID) {
+    val kind = "Syntax"
+    val msg = hl"${symbol.showLocated} has changed semantics in version $migrationVersion"
+    val explanation = {
+      hl"""The ${symbol.showLocated} is marked with ${"@migration"} indicating it has changed semantics
+          |between versions and the ${"-Xmigration"} settings is used to warn about constructs
+          |whose behavior may have changed since version change."""
+    }
+  }
+
   case class UnableToEmitSwitch()(implicit ctx: Context)
   extends Message(UnableToEmitSwitchID) {
     val kind = "Syntax"

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

@@ -685,17 +685,15 @@ object RefChecks {
     }
     // Similar to deprecation: check if the symbol is marked with @migration
     // indicating it has changed semantics between versions.
-    if (sym.hasAnnotation(defn.MigrationAnnot) && ctx.settings.Xmigration.value != NoScalaVersion) {
-      val symVersion: scala.util.Try[ScalaVersion] = sym.migrationVersion.get
-      val changed = symVersion match {
-        case scala.util.Success(v) =>
-          ctx.settings.Xmigration.value < v
+    val xMigrationValue = ctx.settings.Xmigration.value
+    if (sym.hasAnnotation(defn.MigrationAnnot) && xMigrationValue != NoScalaVersion) {
+      sym.migrationVersion.get match {
+        case scala.util.Success(symVersion) if xMigrationValue < symVersion=>
+          ctx.warning(SymbolChangedSemanticsInVersion(sym, symVersion), pos)
         case Failure(ex) =>
-          ctx.warning(s"${sym.showLocated} has an unparsable version number: ${ex.getMessage()}", pos)
-          false
+          ctx.warning(SymbolHasUnparsableVersionNumber(sym, ex.getMessage()), pos)
+        case _ =>
       }
-      if (changed)
-        ctx.warning(s"${sym.showLocated} has changed semantics in version $symVersion:\n${sym.migrationMessage.get}")
     }
     /*  (Not enabled yet)
        *  See an explanation of compileTimeOnly in its scaladoc at scala.annotation.compileTimeOnly.