fix: Check if a PolyFunction TypeTree has no ByName parameters

[KacperFKorban]

closes #21652

[mbovel]

I see that there is already Checking.checkPolyFunctionType that is supposed to check that. So I'm wondering: couldn't we just comment the assertion in PolyFunctionOf.apply, and let PostTyper check that? I tried and it seems to work for this specific example:

-- Error: test.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.
1 error found

[KacperFKorban]

Good idea, less code is always better. I'm not sure about removing the assertion entirely. These types of assertions are pretty useful so that when something crashes, the error is thrown pretty early and not in an unrelated phase. Maybe just the byname and varargs part?

[KacperFKorban]

Hmmm, actually it doesn't work for me. It compiles without any errors by default and crashed after some later phase with -Ycheck:all

[mbovel]

Strange, have we tried the same change?

➜  ~/dotty git:(main) ✗ cat 21671.scala    
def k: [A] => (=> A) => A = // error
   [A] => a => a
➜  ~/dotty git:(main) ✗ git rev-parse HEAD    
c33276637e39e71c13e049d907b239d03a98ad5d
➜  ~/dotty git:(main) ✗ git --no-pager diff           
diff --git a/compiler/src/dotty/tools/dotc/core/Definitions.scala b/compiler/src/dotty/tools/dotc/core/Definitions.scala
index f95bb3cea3..c5b7aa5897 100644
--- a/compiler/src/dotty/tools/dotc/core/Definitions.scala
+++ b/compiler/src/dotty/tools/dotc/core/Definitions.scala
@@ -1221,7 +1221,7 @@ 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")
+      //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.
➜  ~/dotty git:(main) ✗ sbt "scalac 21671.scala"
...
-- Error: 21671.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.
...

[KacperFKorban]

Oooh, actually nevermind. It works, but the isValidPolyFunctionInfo is reused in Checking, so just removing the by-name and vararg checks doesn't work.