#1589: add dedicated error message for missing companion for static members

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

@@ -119,6 +119,7 @@ public enum ErrorMessageID {
     CyclicInheritanceID,
     UnableToExtendSealedClassID,
     UnableToEmitSwitchID,
+    MissingCompanionForStaticID,
     ;
 
     public int errorNumber() {

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

@@ -2001,4 +2001,11 @@ object messages {
           |- there are less than three cases"""
     }
   }
+
+  case class MissingCompanionForStatic(member: Symbol)(implicit ctx: Context) extends Message(MissingCompanionForStaticID) {
+    val msg = hl"${member.owner} does not have a companion class"
+    val kind = "Syntax"
+    val explanation =
+      hl"An object that contains ${"@static"} members must have a companion class."
+  }
 }

compiler/src/dotty/tools/dotc/transform/CheckStatic.scala

@@ -11,7 +11,8 @@ import Flags._
 import Contexts.Context
 import Symbols._
 import Constants._
-import Denotations._, SymDenotations._
+import Denotations._
+import SymDenotations._
 import Decorators.StringInterpolators
 import dotty.tools.dotc.ast.tpd
 import dotty.tools.dotc.core.Annotations.ConcreteAnnotation
@@ -21,7 +22,7 @@ import Names.Name
 import NameOps._
 import Decorators._
 import TypeUtils._
-import reporting.diagnostic.messages.StaticFieldsOnlyAllowedInObjects
+import reporting.diagnostic.messages.{MissingCompanionForStatic, StaticFieldsOnlyAllowedInObjects}
 
 /** A transformer that check that requirements of Static fields\methods are implemented:
   *  1. Only objects can have members annotated with `@static`
@@ -57,7 +58,7 @@ class CheckStatic extends MiniPhase {
         def clashes = companion.asClass.membersNamed(defn.name)
 
         if (!companion.exists) {
-          ctx.error("object that contains @static members should have companion class", defn.pos)
+          ctx.error(MissingCompanionForStatic(defn.symbol), defn.pos)
         } else if (clashes.exists) {
           ctx.error("companion classes cannot define members with same name as @static member", defn.pos)
          } else if (defn.symbol.is(Flags.Mutable) && companion.is(Flags.Trait)) {

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

@@ -1249,4 +1249,17 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       val CyclicInheritance(symbol, _) :: Nil = messages
       assertEquals("class A", symbol.show)
     }
+
+  @Test def missingCompanionForStatic =
+    checkMessagesAfter("checkStatic") {
+      """
+        |object Foo {
+        |  @annotation.static def bar(): Unit = ()
+        |}
+      """.stripMargin
+    }.expect { (itcx, messages) =>
+      implicit val ctx: Context = itcx
+      val MissingCompanionForStatic(member) = messages.head
+      assertEquals(member.show, "method bar")
+    }
 }