Improve @varargs errors and fix some doc

Author: TheElectronWill

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

@@ -128,15 +128,15 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
                       |  - there must be a single repeated parameter
                       |  - it must be the last argument in the last parameter list
                       |""".stripMargin,
-              tree.sourcePos)
+              sym.sourcePos)
             tree
           else
             addVarArgsBridge(tree, isOverride)
         else
           tree
       else
         if hasAnnotation then
-          ctx.error("A method without repeated parameters cannot be annotated with @varargs", tree.sourcePos)
+          ctx.error("A method without repeated parameters cannot be annotated with @varargs", sym.sourcePos)
         tree
     }
 
@@ -156,7 +156,8 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
       lastp.last.isRepeatedParam
     case pt: PolyType =>
       isValidJavaVarArgs(pt.resultType)
-    case _ => false
+    case _ =>
+      throw new Exception("Match error in @varargs bridge logic. This should not happen, please open an issue " + tp)
 
 
   /** Add a Java varargs bridge
@@ -179,22 +180,23 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
     // although it's not strictly necessary for overrides
     // (but it is for non-overrides)
     val flags = ddef.symbol.flags | JavaVarargs
+
+    // The java-compatible bridge symbol
     val bridge = original.copy(
         // non-overrides cannot be synthetic otherwise javac refuses to call them
         flags = if javaOverride then flags | Artifact else flags,
         info = toJavaVarArgs(ddef.symbol.info)
-      ).enteredAfter(thisPhase).asTerm
+      ).asTerm
 
-    val bridgeDenot = bridge.denot
     currentClass.info.member(bridge.name).alternatives.find { s =>
-      s.matches(bridgeDenot) &&
-      !(s.asSymDenotation.is(JavaDefined) && javaOverride)
+      s.matches(bridge) &&
+      !(javaOverride && s.asSymDenotation.is(JavaDefined))
     } match
       case Some(conflict) =>
-        ctx.error(s"@varargs produces a forwarder method that conflicts with ${conflict.showDcl}", ddef.sourcePos)
-        EmptyTree
+        ctx.error(s"@varargs produces a forwarder method that conflicts with ${conflict.showDcl}", original.sourcePos)
+        ddef
       case None =>
-        val bridgeDef = polyDefDef(bridge, trefs => vrefss => {
+        val bridgeDef = polyDefDef(bridge.enteredAfter(thisPhase), trefs => vrefss => {
           val init :+ (last :+ vararg) = vrefss
           // Can't call `.argTypes` here because the underlying array type is of the
           // form `Array[? <: SomeType]`, so we need `.argInfos` to get the `TypeBounds`.
@@ -213,16 +215,16 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
 
   /** Convert type from Scala to Java varargs method */
   private def toJavaVarArgs(tp: Type)(using Context): Type = tp match
-      case tp: PolyType =>
-        tp.derivedLambdaType(tp.paramNames, tp.paramInfos, toJavaVarArgs(tp.resultType))
-      case tp: MethodType =>
-        tp.resultType match
-          case m: MethodType => // multiple param lists
-            tp.derivedLambdaType(tp.paramNames, tp.paramInfos, toJavaVarArgs(m))
-          case _ =>
-            val init :+ last = tp.paramInfos
-            val vararg = varargArrayType(last)
-            tp.derivedLambdaType(tp.paramNames, init :+ vararg, tp.resultType)
+    case tp: PolyType =>
+      tp.derivedLambdaType(tp.paramNames, tp.paramInfos, toJavaVarArgs(tp.resultType))
+    case tp: MethodType =>
+      tp.resultType match
+        case m: MethodType => // multiple param lists
+          tp.derivedLambdaType(tp.paramNames, tp.paramInfos, toJavaVarArgs(m))
+        case _ =>
+          val init :+ last = tp.paramInfos
+          val vararg = varargArrayType(last)
+          tp.derivedLambdaType(tp.paramNames, init :+ vararg, tp.resultType)
 
   /** Translate a repeated type T* to an `Array[? <: Upper]`
    *  such that it is compatible with java varargs.

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

@@ -314,8 +314,8 @@ object GenericSignatures {
     *  - If the list contains one or more occurrences of scala.Array with
     *    type parameters El1, El2, ... then the dominator is scala.Array with
     *    type parameter of intersectionDominator(List(El1, El2, ...)).           <--- @PP: not yet in spec.
-    *  - Otherwise, the list is reduced to a subsequence containing only types
-    *    which are not subtypes of other listed types (the span.)
+    *  - Otherwise, the list is reduced to a subsequence containing only the
+    *    types that are not supertypes of other listed types (the span.)
     *  - If the span is empty, the dominator is Object.
     *  - If the span contains a class Tc which is not a trait and which is
     *    not Object, the dominator is Tc.                                        <--- @PP: "which is not Object" not in spec.