Fix #7103: Accept backquoted as as the name of a given

Author: odersky

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

@@ -957,7 +957,7 @@ object desugar {
           case Some(DefDef(name, _, (vparam :: _) :: _, _, _)) =>
             s"${name}_of_${inventTypeName(vparam.tpt)}"
           case _ =>
-            ctx.error(i"anonymous instance must have `for` part or must define at least one extension method", impl.sourcePos)
+            ctx.error(i"anonymous instance must have `as` part or must define at least one extension method", impl.sourcePos)
             nme.ERROR.toString
         }
       else

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

@@ -2873,7 +2873,7 @@ object Parsers {
      */
     def instanceDef(newStyle: Boolean, start: Offset, mods: Modifiers, instanceMod: Mod) = atSpan(start, nameStart) {
       var mods1 = addMod(mods, instanceMod)
-      val name = if (isIdent && (!newStyle || in.name != nme.as)) ident() else EmptyTermName
+      val name = if (isIdent && !(newStyle && isIdent(nme.as))) ident() else EmptyTermName
       val tparams = typeParamClauseOpt(ParamOwner.Def)
       var leadingParamss =
         if (in.token == LPAREN)

tests/pos/i7103.scala

@@ -0,0 +1,3 @@
+class X
+object X extends X
+given `as` as X = X