Handle right-associative extension operators

Author: odersky

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

@@ -208,10 +208,20 @@ abstract class Positioned extends Product {
         // Leave out tparams, they are copied with wrong positions from parent class
         check(tree.mods)
         check(tree.vparamss)
-      case tree: DefDef if tree.mods.is(Extension) && tree.vparamss.nonEmpty =>
-        check(tree.vparamss.head)
-        check(tree.tparams)
-        check(tree.vparamss.tail)
+      case tree: DefDef if tree.mods.is(Extension) =>
+        tree.vparamss match {
+          case vparams1 :: vparams2 :: rest if !isLeftAssoc(tree.name) =>
+            check(vparams2)
+            check(tree.tparams)
+            check(vparams1)
+            check(rest)
+          case vparams1 :: rest =>
+            check(vparams1)
+            check(tree.tparams)
+            check(rest)
+          case _ =>
+            check(tree.tparams)
+        }
         check(tree.tpt)
         check(tree.rhs)
       case _ =>

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

@@ -2259,7 +2259,12 @@ object Parsers {
         val mods1 = addFlag(mods, flags)
         val name = ident()
         val tparams = typeParamClauseOpt(ParamOwner.Def)
-        val vparamss = leadingParamss ::: paramClauses(ofMethod = true)
+        val vparamss = paramClauses(ofMethod = true) match {
+          case rparams :: rparamss if leadingParamss.nonEmpty && !isLeftAssoc(name) =>
+            rparams :: leadingParamss ::: rparamss
+          case rparamss =>
+            leadingParamss ::: rparamss
+        }
         var tpt = fromWithinReturnType {
           if (in.token == SUBTYPE && mods.is(Inline)) {
             in.nextToken()