KindProjectorScala3TypeLambda

build.sbt

@@ -191,6 +191,8 @@ lazy val inputOutputCommon = Def.settings(
     scalaBinaryVersion.value match {
       case "2.13" =>
         Seq("-Wconf:cat=scala3-migration:info")
+      case "3" =>
+        Seq("-Ykind-projector")
       case _ =>
         Nil
     }

input/src/main/scala-3/fix/KindProjectorScala3TypeLambdaTest.scala

@@ -0,0 +1,10 @@
+/*
+rule = KindProjectorScala3TypeLambda
+ */
+package fix
+
+trait KindProjectorScala3TypeLambdaTest {
+  type X1[b] = λ[a => Either[a, b]]
+
+  def f: X1[Int][String] = Right(2).withLeft[String]
+}

output/src/main/scala-3/fix/KindProjectorScala3TypeLambdaTest.scala

@@ -0,0 +1,7 @@
+package fix
+
+trait KindProjectorScala3TypeLambdaTest {
+  type X1[b] = [a] =>> Either[a, b]
+
+  def f: X1[Int][String] = Right(2).withLeft[String]
+}

rules/src/main/scala/fix/KindProjectorScala3TypeLambda.scala

@@ -0,0 +1,25 @@
+package fix
+
+import scala.meta.Type
+import scala.meta.XtensionCollectionLikeUI
+import scalafix.Patch
+import scalafix.v1.SyntacticDocument
+import scalafix.v1.SyntacticRule
+import scalafix.v1.XtensionSeqPatch
+
+class KindProjectorScala3TypeLambda extends SyntacticRule("KindProjectorScala3TypeLambda") {
+  override def fix(implicit doc: SyntacticDocument): Patch = {
+    doc.tree.collect {
+      case t @ Type.Apply.After_4_6_0(
+            Type.Name("λ"),
+            Type.ArgClause(
+              Type.Function.After_4_6_0(
+                Type.FuncParamClause(args),
+                body
+              ) :: Nil
+            )
+          ) =>
+        Patch.replaceTree(t, args.mkString("[", ", ", s"] =>> ${body}"))
+    }.asPatch
+  }
+}