Simplify ByName handling in initialization checker

Since ByName nodes are now explicit, no need to introduce them separately in the
checker.

compiler/src/dotty/tools/dotc/transform/init/Semantic.scala

@@ -1054,19 +1054,18 @@ object Semantic {
     exprs.map { expr => eval(expr, thisV, klass) }
 
   /** Evaluate arguments of methods */
-  def evalArgs(args: List[Arg], thisV: Ref, klass: ClassSymbol): Contextual[(List[Error], List[ArgInfo])] =
+  def evalArgs(args: List[Tree], thisV: Ref, klass: ClassSymbol): Contextual[(List[Error], List[ArgInfo])] =
     val errors = new mutable.ArrayBuffer[Error]
     val argInfos = new mutable.ArrayBuffer[ArgInfo]
     args.foreach { arg =>
-      val res =
-        if arg.isByName then
-          val fun = Fun(arg.tree, thisV, klass, env)
+      val res = arg match
+        case ByName(_) =>
+          val fun = Fun(arg, thisV, klass, env)
           Result(fun, Nil)
-        else
-          eval(arg.tree, thisV, klass)
-
+        case _ =>
+          eval(arg, thisV, klass)
       errors ++= res.errors
-      argInfos += ArgInfo(res.value, arg.tree)
+      argInfos += ArgInfo(res.value, arg)
     }
     (errors.toList, argInfos.toList)
 
@@ -1491,31 +1490,17 @@ object Semantic {
     case hklambda: HKTypeLambda => typeRefOf(hklambda.resType)
   }
 
-  opaque type Arg  = Tree | ByNameArg  // ^^^ can be simplified
-  case class ByNameArg(tree: Tree)
-
-  extension (arg: Arg)
-    def isByName = arg.isInstanceOf[ByNameArg]
-    def tree: Tree = arg match
-      case t: Tree      => t
-      case ByNameArg(t) => t
-
   object Call {
 
     private def isDelayed(tp: Type)(using Context) =
       tp.isInstanceOf[MethodicType] || tp.isByName
 
-    def unapply(tree: Tree)(using Context): Option[(Tree, List[List[Arg]])] =
+    def unapply(tree: Tree)(using Context): Option[(Tree, List[List[Tree]])] =
       tree match
       case Apply(fn, args) =>
-        val argTps = fn.tpe.widen match
-          case mt: MethodType => mt.paramInfos
-        val normArgs: List[Arg] = args.zip(argTps).map { (arg, formal) =>
-          if formal.isByName then ByNameArg(arg.dropByName) else arg
-        }
         unapply(fn) match
-        case Some((ref, args0)) => Some((ref, args0 :+ normArgs))
-        case None => None
+          case Some((ref, args0)) => Some((ref, args0 :+ args))
+          case None => None
 
       case TypeApply(fn, targs) =>
         unapply(fn)
@@ -1527,7 +1512,7 @@ object Semantic {
   }
 
   object NewExpr {
-    def unapply(tree: Tree)(using Context): Option[(TypeRef, New, Symbol, List[List[Arg]])] =
+    def unapply(tree: Tree)(using Context): Option[(TypeRef, New, Symbol, List[List[Tree]])] =
       tree match
       case Call(fn @ Select(newTree: New, init), argss) if init == nme.CONSTRUCTOR =>
         val tref = typeRefOf(newTree.tpe)