Fix parser not recognizing using erased X

By parsing the flags of the first parameter eagerly.

Author: natsukagami

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

@@ -2322,10 +2322,11 @@ object Parsers {
         if in.token == RPAREN then
           Nil
         else
-          var mods1 = mods
-          if isErased then mods1 = addModifier(mods1)
           try
-            commaSeparated(() => binding(mods1))
+            commaSeparated(() =>
+              val mods1 = if isErased then addModifier(mods) else mods
+              binding(mods1)
+            )
           finally
             accept(RPAREN)
       else {
@@ -3249,14 +3250,22 @@ object Parsers {
             if prefix && !isIdent(nme.using) && !isIdent(nme.erased) then param() :: Nil
             else
               paramMods()
+              val firstParamMod =
+                var mods = EmptyModifiers
+                if isErased then mods = addModifier(mods)
+                if in.name == nme.inline then mods = addModifier(mods)
+                mods
               if givenOnly && !impliedMods.is(Given) then
                 syntaxError(em"`using` expected")
               val isParams =
                 !impliedMods.is(Given)
                 || startParamTokens.contains(in.token)
-                || isIdent && (in.name == nme.inline || in.name == nme.erased || in.lookahead.isColon)
-              if isParams then commaSeparated(() => param())
-              else contextTypes(ofClass, nparams, impliedMods)
+                || isIdent && in.lookahead.isColon
+              (if isParams then commaSeparated(() => param())
+              else contextTypes(ofClass, nparams, impliedMods)) match {
+                case Nil => Nil
+                case (h :: t) => h.withAddedFlags(firstParamMod.flags) :: t
+              }
           checkVarArgsRules(clause)
           clause
       }