Tweak zipWithConserve to eliminate 3 casts

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

@@ -144,13 +144,13 @@ object Decorators {
      *  `xs` to themselves. Also, it is required that `ys` is at least
      *  as long as `xs`.
      */
-    def zipWithConserve[U](ys: List[U])(f: (T, U) => T): List[T] =
+    def zipWithConserve[U, V <: T](ys: List[U])(f: (T, U) => V): List[V] =
       if (xs.isEmpty || ys.isEmpty) Nil
       else {
         val x1 = f(xs.head, ys.head)
         val xs1 = xs.tail.zipWithConserve(ys.tail)(f)
-        if ((x1.asInstanceOf[AnyRef] eq xs.head.asInstanceOf[AnyRef]) &&
-            (xs1 eq xs.tail)) xs
+        if (x1.asInstanceOf[AnyRef] eq xs.head.asInstanceOf[AnyRef]) && (xs1 eq xs.tail)
+          then xs.asInstanceOf[List[V]]
         else x1 :: xs1
       }
 

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

@@ -71,9 +71,7 @@ case class Signature(paramsSig: List[ParamSig], resSig: TypeName) {
     else if (!this.paramsSig.hasSameLengthAs(that.paramsSig)) that
     else {
       val mapped = Signature(
-          // DOTTY: we shouldn't have to explicitly pass a type argument to `update`,
-          // see https://github.com/lampepfl/dotty/issues/4867
-          this.paramsSig.zipWithConserve(that.paramsSig)(update[ParamSig]),
+          this.paramsSig.zipWithConserve(that.paramsSig)(update),
           update(this.resSig, that.resSig))
       if (mapped == this) this else mapped
     }

compiler/src/dotty/tools/dotc/core/tasty/TastyUnpickler.scala

@@ -54,9 +54,7 @@ class TastyUnpickler(reader: TastyReader) {
     val end = start + length
     def readSignedRest(original: TermName, target: TermName): TermName =
       val result = readName().toTypeName
-      // DOTTY: we shouldn't have to give an explicit type to paramsSig,
-      // see https://github.com/lampepfl/dotty/issues/4867
-      val paramsSig: List[Signature.ParamSig] = until(end)(readParamSig())
+      val paramsSig = until(end)(readParamSig())
       val sig = Signature(paramsSig, result)
       SignedName(original, sig, target)
 

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

@@ -825,7 +825,6 @@ object Erasure {
 
           val args0 = outers ::: ownArgs
           val args1 = args0.zipWithConserve(xmt.paramInfos)(typedExpr)
-            .asInstanceOf[List[Tree]]
 
           def mkApply(finalFun: Tree, finalArgs: List[Tree]) =
             val app = untpd.cpy.Apply(tree)(finalFun, finalArgs)

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

@@ -110,7 +110,7 @@ class ReTyper(nestingLevel: Int = 0) extends Typer(nestingLevel) with ReChecking
 
   override def handleUnexpectedFunType(tree: untpd.Apply, fun: Tree)(using Context): Tree = fun.tpe match {
     case mt: MethodType =>
-      val args: List[Tree] = tree.args.zipWithConserve(mt.paramInfos)(typedExpr(_, _)).asInstanceOf[List[Tree]]
+      val args: List[Tree] = tree.args.zipWithConserve(mt.paramInfos)(typedExpr)
       assignType(untpd.cpy.Apply(tree)(fun, args), fun, args)
     case _ =>
       super.handleUnexpectedFunType(tree, fun)

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

@@ -1969,7 +1969,7 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
             }
           else desugaredArg.withType(UnspecifiedErrorType)
         }
-        args.zipWithConserve(tparams)(typedArg(_, _)).asInstanceOf[List[Tree]]
+        args.zipWithConserve(tparams)(typedArg)
       }
       val paramBounds = tparams.lazyZip(args).map {
         case (tparam, untpd.WildcardTypeBoundsTree()) =>