fix: Check if a PolyFunction TypeTree has no ByName parameters

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/reporting/messages.scala

@@ -1829,12 +1829,11 @@ class NotAPath(tp: Type, usage: String)(using Context) extends TypeMsg(NotAPathI
     if sym.isAllOf(Flags.InlineParam) then
       i"""
          |Inline parameters are not considered immutable paths and cannot be used as
-         |singleton types. 
-         | 
+         |singleton types.
+         |
          |Hint: Removing the `inline` qualifier from the `${sym.name}` parameter
          |may help resolve this issue."""
     else ""
-
 
 class WrongNumberOfParameters(tree: untpd.Tree, foundCount: Int, pt: Type, expectedCount: Int)(using Context)
   extends SyntaxMsg(WrongNumberOfParametersID) {

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

@@ -1914,7 +1914,6 @@ class Typer(@constructorOnly nestingLevel: Int = 0) extends Namer
       .showing(i"desugared fun $tree --> $desugared with pt = $pt", typr)
   }
 
-
   def typedPolyFunction(tree: untpd.PolyFunction, pt: Type)(using Context): Tree =
     val tree1 = desugar.normalizePolyFunction(tree)
     if (ctx.mode is Mode.Type) typed(desugar.makePolyFunctionType(tree1), pt)

tests/neg/21538.check

@@ -3,8 +3,8 @@
   |                                             ^^^^^^^^^^
   |                                      (value : V) is not a valid singleton type, since it is not an immutable path
   |                                      Inline parameters are not considered immutable paths and cannot be used as
-  |                                      singleton types. 
-  |                                       
+  |                                      singleton types.
+  |
   |                                      Hint: Removing the `inline` qualifier from the `value` parameter
   |                                      may help resolve this issue.
   |

tests/neg/i21652.check

@@ -0,0 +1,4 @@
+-- Error: tests/neg/i21652.scala:1:8 -----------------------------------------------------------------------------------
+1 |def k: [A] => (=> A) => A = // error
+  |        ^^^^^^^^^^^^^^^^^
+  |Implementation restriction: PolyFunction apply must have exactly one parameter list and optionally type arguments. No by-name nor varags are allowed.

tests/neg/i21652.scala

@@ -0,0 +1,2 @@
+def k: [A] => (=> A) => A = // error
+  [A] => a => a