Correctly specify argument type when in Java varargs bridge

Author: abeln

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

@@ -125,6 +125,13 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
    *  @return  a thicket consisting of `ddef` and a varargs bridge method
    *           which overrides the Java varargs method JM from this phase on
    *           and forwards to `ddef`.
+   *
+   *  A bridge is necessary because the following hold
+   *    - the varargs in `ddef` will change from `RepeatedParam[T]` to `Seq[T]` after this phase
+   *    - _but_ the callers of `ddef` expect its varargs to be changed to `Array[_ <: T]`, since it overrides
+   *      a Java varargs
+   *  The solution is to add a "bridge" method that converts its argument from `Array[_ <: T]` to `Seq[T]` and
+   *  forwards it to `ddef`.
    */
   private def addVarArgsBridge(ddef: DefDef)(implicit ctx: Context): Tree = {
     val original = ddef.symbol.asTerm
@@ -133,7 +140,9 @@ class ElimRepeated extends MiniPhase with InfoTransformer { thisPhase =>
       info = toJavaVarArgs(ddef.symbol.info)).enteredAfter(thisPhase).asTerm
     val bridgeDef = polyDefDef(bridge, trefs => vrefss => {
       val (vrefs :+ varArgRef) :: vrefss1 = vrefss
-      val elemtp = varArgRef.tpe.widen.argTypes.head
+      // Can't call `.argTypes` here because the underlying array type is of the
+      // form `Array[_ <: SomeType]`, so we need `.argInfos` to get the `TypeBounds`.
+      val elemtp = varArgRef.tpe.widen.argInfos.head
       ref(original.termRef)
         .appliedToTypes(trefs)
         .appliedToArgs(vrefs :+ tpd.wrapArray(varArgRef, elemtp))