WIP try to improve error messages

Author: Jendrik Wenke

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

@@ -2082,7 +2082,14 @@ object messages {
 
   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 msg = {
+      val details = decl.asTerm.signature.matchDegree(previousSymbol.asTerm.signature) match {
+        case Signature.NoMatch => "" // matchDegree also returns NoMatch if one of the terms is not a method
+        case Signature.ParamMatch => "\nOverloads with equal parameter types but different return types are not allowed."
+        case Signature.FullMatch => "\nThe definitions have the same signature after erasure."
+      }
+      hl"${decl.showLocated} is already defined as ${previousSymbol.showDcl} in line ${previousSymbol.pos.line}." + details
+    }
     val explanation = ""
   }
 }

tests/neg/doubleDefinition.scala

@@ -0,0 +1,29 @@
+trait A
+trait B
+
+class Test1 {
+	def foo(x: List[A]): Function1[A, A] = ???
+	def foo(x: List[B]): Function2[B, B, B] = ???
+	// ok, different jvm signature
+}
+
+
+class Test2 {
+
+
+
+	def foo(
+		       x: List[A]
+	       ): Function1[A,
+		A] =
+		???
+	def foo(x: List[B]): Function1[B, B] = ??? // error: same jvm signature
+	// scalac calls this "have same type after erasure"
+}
+
+
+class Test3 {
+	// overload with same argument type, but different return types
+	def foo(x: List[A]): Function1[A, A] = ???
+	def foo(x: List[A]): Function2[B, B, B] = ??? // error
+}