Generate implied @infix annotation as untyped tree

Generating it as a typed tree will lead to a duplicate class `annotation.infix`
on bootstrap.

Also, update a check file to report the inserted @infix annotation.

Author: odersky

compiler/src/dotty/tools/dotc/ast/untpd.scala

@@ -445,6 +445,7 @@ object untpd extends Trees.Instance[Untyped] with UntypedTreeInfo {
 
   def rootDot(name: Name)(implicit src: SourceFile): Select = Select(Ident(nme.ROOTPKG), name)
   def scalaDot(name: Name)(implicit src: SourceFile): Select = Select(rootDot(nme.scala_), name)
+  def scalaAnnotationDot(name: Name)(using SourceFile): Select = Select(scalaDot(nme.annotation), name)
   def scalaUnit(implicit src: SourceFile): Select = scalaDot(tpnme.Unit)
   def scalaAny(implicit src: SourceFile): Select = scalaDot(tpnme.Any)
   def javaDotLangDot(name: Name)(implicit src: SourceFile): Select = Select(Select(Ident(nme.java), nme.lang), name)

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

@@ -486,6 +486,7 @@ object StdNames {
     val implicitly: N           = "implicitly"
     val in: N                   = "in"
     val inline: N               = "inline"
+    val infix: N                = "infix"
     val info: N                 = "info"
     val inlinedEquals: N        = "inlinedEquals"
     val internal: N             = "internal"

compiler/src/dotty/tools/dotc/parsing/Parsers.scala

@@ -3217,7 +3217,7 @@ object Parsers {
         val ident = termIdent()
         val name = ident.name.asTermName
         if isInfix && !name.isOperatorName then
-          val infixAnnot = Apply(wrapNew(ref(defn.InfixAnnot.typeRef)), Nil)
+          val infixAnnot = Apply(wrapNew(scalaAnnotationDot(tpnme.infix)), Nil)
               .withSpan(Span(start, start))
           mods1 = mods1.withAddedAnnotation(infixAnnot)
         val tparams =

tests/printing/i620.check

@@ -11,7 +11,7 @@ package O {
         protected[A] def f: Int = 0
         def g: Int = 0
         def g1(t: Int): Int = 0
-        def (c: D.this.C) g1: Int = 0
+        @_root_.scala.annotation.infix() def (c: D.this.C) g1: Int = 0
       }
       private[D] class E() extends Object() {}
       private[this] class F() extends Object() {}