Fix application with multiple type params

Author: Sporarum

compiler/src/dotty/tools/dotc/typer/Applications.scala

@@ -1085,8 +1085,8 @@ trait Applications extends Compatibility {
     val typedArgs = if (isNamed) typedNamedArgs(tree.args) else tree.args.mapconserve(typedType(_))
     record("typedTypeApply")
     typedExpr(tree.fun, PolyProto(typedArgs, pt)) match {
-      case _: TypeApply if !ctx.isAfterTyper =>
-        errorTree(tree, "illegal repeated type application")
+      /* case _: TypeApply if !ctx.isAfterTyper =>  //TODO: assess removing this is okay
+        errorTree(tree, "illegal repeated type application") */
       case typedFn =>
         typedFn.tpe.widen match {
           case pt: PolyType =>

tests/pos/_typeInterweaving/higherKindedReturn.scala

@@ -0,0 +1,21 @@
+def f_4[T]: [U] => T => T = ???
+def f_3[T][U]: T => T = ???
+def f_2[T][U](): T => T = ???
+def f_1[T <: Int][U <: String](): T => T = ???
+def f0[T <: Int][U <: String]: T => T = ???
+def f1[T <: Int][U <: String]: [X <: Unit] => X => X = ???
+def f2[T <: Int][U <: String](): [X <: Unit] => X => X = ???
+def f3[T <: Int][U <: String]()[X <: Unit]: X => X = ???
+
+@main def test = {
+    f_4[Int][String] //only one that works when lines 1088 to 1089 of Applications.scala are uncommented
+    f_3[Int][String]
+    f_2[Int][String]()
+    f_1[Int][String]()
+    f0[Int][String]
+    f1[Int][String]
+    f1[Int][Unit]
+    f1[Int][String][Unit]
+    f2[Int]()[Unit]
+    f3[Int]()[Unit]
+}