Accept as also without preceding name or parameters

This aligns the Parser with the syntax.

Author: odersky

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

@@ -3495,7 +3495,7 @@ object Parsers {
           then paramClauses(givenOnly = true)
           else Nil
         newLinesOpt()
-        if !name.isEmpty || !tparams.isEmpty || !vparamss.isEmpty then
+        if isIdent(nme.as) || !name.isEmpty || !tparams.isEmpty || !vparamss.isEmpty then
           accept(nme.as)
         def givenAlias(tpt: Tree) =
           accept(EQUALS)

tests/run/instances-anonymous.scala

@@ -63,7 +63,7 @@ object Test extends App {
     val minimum: T
   }
 
-  given Ord[Int] {
+  given as Ord[Int] {
     def (x: Int).compareTo(y: Int) =
       if (x < y) -1 else if (x > y) +1 else 0
     val minimum = Int.MinValue