Let the poly functions validation be handled in Checking

compiler/src/dotty/tools/dotc/core/Definitions.scala

@@ -1221,7 +1221,6 @@ class Definitions {
 
     /** Creates a refined `PolyFunction` with an `apply` method with the given info. */
     def apply(mt: MethodOrPoly)(using Context): Type =
-      assert(isValidPolyFunctionInfo(mt), s"Not a valid PolyFunction refinement: $mt")
       RefinedType(PolyFunctionClass.typeRef, nme.apply, mt)
 
     /** Matches a refined `PolyFunction` type and extracts the apply info.

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

@@ -1914,26 +1914,10 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       .showing(i"desugared fun $tree --> $desugared with pt = $pt", typr)
   }
 
-  /** Check that the PolyFunction doesn't have by-name parameters.
-   *  Return the unchanged tree if it's valid, or EmptyTree otherwise.
-   */
-  private def checkPolyTypeTree(tree: untpd.Tree)(using Context): untpd.Tree =
-    val untpd.PolyFunction(tparams: List[untpd.TypeDef] @unchecked, fun @ untpd.Function(vparamTypes, res)) = tree: @unchecked
-    var tree1 = tree
-    vparamTypes.foreach:
-      case t: ByNameTypeTree =>
-        report.error("By-name parameters are not supported in Polymorphic Functions", t.srcPos)
-        tree1 = untpd.EmptyTree
-      case _ =>
-    tree1
-
   def typedPolyFunction(tree: untpd.PolyFunction, pt: Type)(using Context): Tree =
     val tree1 = desugar.normalizePolyFunction(tree)
-    checkPolyTypeTree(tree1) match
-      case tree2: untpd.PolyFunction =>
-        if (ctx.mode is Mode.Type) typed(desugar.makePolyFunctionType(tree2), pt)
-        else typedPolyFunctionValue(tree2, pt)
-      case untpd.EmptyTree => TypeTree(NoType)
+    if (ctx.mode is Mode.Type) typed(desugar.makePolyFunctionType(tree1), pt)
+    else typedPolyFunctionValue(tree1, pt)
 
   def typedPolyFunctionValue(tree: untpd.PolyFunction, pt: Type)(using Context): Tree =
     val untpd.PolyFunction(tparams: List[untpd.TypeDef] @unchecked, fun) = tree: @unchecked

tests/neg/i21652.check

@@ -1,4 +1,4 @@
--- Error: tests/neg/i21652.scala:1:15 ----------------------------------------------------------------------------------
+-- Error: tests/neg/i21652.scala:1:8 -----------------------------------------------------------------------------------
 1 |def k: [A] => (=> A) => A = // error
-  |               ^^^^
-  |               By-name parameters are not supported in Polymorphic Functions
+  |        ^^^^^^^^^^^^^^^^^
+  |Implementation restriction: PolyFunction apply must have exactly one parameter list and optionally type arguments. No by-name nor varags are allowed.