Add error message about double definition (#1589)

Author: Jendrik Wenke

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

@@ -127,6 +127,7 @@ public enum ErrorMessageID {
     PolymorphicMethodMissingTypeInParentID,
     ParamsNoInlineID,
     JavaSymbolIsNotAValueID,
+    DoubleDeclarationID,
     ;
 
     public int errorNumber() {

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

@@ -2079,4 +2079,10 @@ object messages {
     }
     val explanation = ""
   }
+
+  case class DoubleDeclaration(decl: Symbol, previousSymbol: Symbol)(implicit ctx: Context) extends Message(DoubleDeclarationID) {
+    val kind = "Duplicate Symbol"
+    val msg = hl"$decl is already defined as $previousSymbol${previousSymbol.showExtendedLocation}"
+    val explanation = ""
+  }
 }

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

@@ -627,7 +627,7 @@ trait Checking {
             def explanation =
               if (!decl.isRealMethod) ""
               else "\n(the definitions have matching type signatures)"
-            ctx.error(em"$decl is already defined as $other$ofType$explanation", decl.pos)
+            ctx.error(DoubleDeclaration(decl, other), decl.pos)
           }
           if (decl is Synthetic) doubleDefError(other, decl)
           else doubleDefError(decl, other)

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

@@ -2,12 +2,10 @@ package dotty.tools
 package dotc
 package reporting
 
-import core.Contexts.Context
-import diagnostic.messages._
-import dotty.tools.dotc.core.Flags
-import dotty.tools.dotc.core.Flags.FlagSet
+import dotty.tools.dotc.core.Contexts.Context
 import dotty.tools.dotc.core.Types.WildcardType
 import dotty.tools.dotc.parsing.Tokens
+import dotty.tools.dotc.reporting.diagnostic.messages._
 import org.junit.Assert._
 import org.junit.Test
 
@@ -1293,4 +1291,19 @@ class ErrorMessagesTests extends ErrorMessagesTest {
 
       assert(ctx.reporter.hasErrors)
     }
+
+  @Test def typeDoubleDeclaration =
+    checkMessagesAfter("frontend") {
+      """
+        |class Foo {
+        |  val a = 1
+        |  val a = 2
+        |}
+      """.stripMargin
+    }.expect { (ictx, messages) =>
+      implicit val ctx: Context = ictx
+      assertMessageCount(1, messages)
+      val DoubleDeclaration(symbol, previousSymbol) :: Nil = messages
+      assertEquals(symbol.name.mangledString, "a")
+  }
 }