Added error message for Symbol is not a value #1589

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

@@ -124,6 +124,7 @@ public enum ErrorMessageID {
     SymbolChangedSemanticsInVersionID,
     UnableToEmitSwitchID,
     MissingCompanionForStaticID,
+    JavaSymbolIsNotAValueID,
     ;
 
     public int errorNumber() {

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

@@ -208,7 +208,7 @@ object messages {
 
   case class WildcardOnTypeArgumentNotAllowedOnNew()(implicit ctx: Context)
   extends Message(WildcardOnTypeArgumentNotAllowedOnNewID) {
-    val kind = "syntax"
+    val kind = "Syntax"
     val msg = "type argument must be fully defined"
 
     val code1 =
@@ -2064,4 +2064,25 @@ object messages {
     val explanation =
       hl"An object that contains ${"@static"} members must have a companion class."
   }
+
+  case class JavaSymbolIsNotAValue(symbol: Symbol)(implicit ctx: Context) extends Message(JavaSymbolIsNotAValueID) {
+    val msg = hl"$symbol is not a value"
+    val kind = "Type Mismatch"
+    val explanation = {
+      val javaCodeExample = """class A {public static int a() {return 1;}}"""
+
+      val scalaCodeExample =
+        """val objectA = A     // This does not compile
+          |val aResult = A.a() // This does compile""".stripMargin
+
+      hl"""Java statics and packages cannot be used as a value.
+          |For Java statics consider the following Java example:
+          |
+          |$javaCodeExample
+          |
+          |When used from Scala:
+          |
+          |$scalaCodeExample"""
+    }
+  }
 }

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

@@ -523,7 +523,7 @@ trait Checking {
       val sym = tree.tpe.termSymbol
       // The check is avoided inside Java compilation units because it always fails
       // on the singleton type Module.type.
-      if ((sym is Package) || ((sym is JavaModule) && !ctx.compilationUnit.isJava)) ctx.error(em"$sym is not a value", tree.pos)
+      if ((sym is Package) || ((sym is JavaModule) && !ctx.compilationUnit.isJava)) ctx.error(JavaSymbolIsNotAValue(sym), tree.pos)
     }
     tree
   }

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

@@ -1277,4 +1277,18 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       val MissingCompanionForStatic(member) = messages.head
       assertEquals(member.show, "method bar")
     }
+
+  @Test def javaSymbolIsNotAValue =
+    checkMessagesAfter("checkStatic") {
+      """
+        |package p
+        |object O {
+        |  val v = p
+        |}
+      """.stripMargin
+    }.expect { (itcx, messages) =>
+      implicit val ctx: Context = itcx
+      val JavaSymbolIsNotAValue(symbol) = messages.head
+      assertEquals(symbol.show, "package p")
+    }
 }