Improve code based on review

Author: nicolasstucki

compiler/src/dotty/tools/dotc/core/PhantomErasure.scala

@@ -7,7 +7,7 @@ import dotty.tools.dotc.core.Types.Type
 
 /** Phantom erasure erases:
  *
- *  - Parameters/arguments are erased removed from the function definition/call in `PhantomArgumentEval`.
+ *  - Parameters/arguments are removed from the function definition/call in `PhantomArgumentEval`.
  *    If the evaluation of the phantom arguments may produce a side effect, these are evaluated and stored in
  *    local `val`s and then the non phantoms are used in the Apply. Phantom `val`s are then erased to
  *    `val ev$i: ErasedPhantom = myPhantom` intended to be optimized away by local optimizations. `myPhantom` could be

compiler/src/dotty/tools/dotc/core/TypeErasure.scala

@@ -401,8 +401,8 @@ class TypeErasure(isJava: Boolean, semiEraseVCs: Boolean, isConstructor: Boolean
       def paramErasure(tpToErase: Type) =
         erasureFn(tp.isJava, semiEraseVCs, isConstructor, wildcardOK)(tpToErase)
       val (names, formals0) =
-        if (tp.paramInfos.forall(!_.isPhantom)) (tp.paramNames, tp.paramInfos)
-        else tp.paramNames.iterator.zip(tp.paramInfos.iterator).filterNot(_._2.isPhantom).toList.unzip
+        if (tp.paramInfos.exists(_.isPhantom)) tp.paramNames.zip(tp.paramInfos).filterNot(_._2.isPhantom).unzip
+        else (tp.paramNames, tp.paramInfos)
       val formals = formals0.mapConserve(paramErasure)
       eraseResult(tp.resultType) match {
         case rt: MethodType =>

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

@@ -477,6 +477,7 @@ object Erasure extends TypeTestsCasts{
                   .withType(defn.ArrayOf(defn.ObjectType))
                 args0 = bunchedArgs :: Nil
               }
+              // Arguments are phantom if an only if the parameters are phantom, guaranteed by the separation of type lattices
               val args1 = args0.filterConserve(arg => !wasPhantom(arg.typeOpt)).zipWithConserve(mt.paramInfos)(typedExpr)
               untpd.cpy.Apply(tree)(fun1, args1) withType mt.resultType
             case _ =>