Added error message for Symbol is not a value #1589

Author: benkobalog

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

@@ -125,6 +125,7 @@ public enum ErrorMessageID {
     UnableToEmitSwitchID,
     MissingCompanionForStaticID,
     PolymorphicMethodMissingTypeInParentID,
+    JavaSymbolIsNotAValueID,
     ;
 
     public int errorNumber() {

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

@@ -2069,4 +2069,26 @@ object messages {
           |$rsym does not override any method in $parentSym. Structural refinement does not allow for
           |polymorphic methods."""
   }
+
+  case class JavaSymbolIsNotAValue(symbol: Symbol)(implicit ctx: Context) extends Message(JavaSymbolIsNotAValueID) {
+    val msg = hl"$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

@@ -1279,4 +1279,18 @@ class ErrorMessagesTests extends ErrorMessagesTest {
       assertEquals("method get", rsym.show)
       assertEquals("class Object", parentSym.show)
     }
+
+  @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")
+    }
 }