Codegen Regression - `function` within recursive function results in null reference exception

Using function within a recursive function iterating over a list results in a null reference exception when matching against the empty list [].

Repro steps

A sample repo has been made: https://github.com/StachuDotNet/function-rec-fsharp-dotnet7-bug-demo/blob/main/Program.fs

let list = [1; 2; 3]
let condition n = n < 3

let dropWhileWithMatch condition list =
  let rec f (l : List<int>) : List<int> =
    match l with
    | [] -> []
    | head :: tail ->
      match condition head with
      | true -> f tail
      | false -> head :: tail

  f list

let dropWhileWithFunction condition list =
  let rec f : List<int> -> List<int> =
    function
    | [] -> []
    | head :: tail ->
      match condition head with
      | true -> f tail
      | false -> head :: tail

  f list


// this runs fine:
let matchResult = dropWhileWithMatch condition list
printfn "Match: %A" matchResult


// and this results in a null reference exception
let functionResult = dropWhileWithFunction condition list
printfn "Function: %A" functionResult
(*
  Unhandled exception. System.NullReferenceException: Object reference not set to an instance of an object.
   at [e-mail](FSharpList`1 _arg1) in /workspaces/function-rec-fsharp-dotnet7-bug-demo/Program.fs:line 18
   at Program.dropWhileWithFunction(FSharpFunc`2 condition, FSharpList`1 list) in /workspaces/function-rec-fsharp-dotnet7-bug-demo/Program.fs:line 21
   at <StartupCode$function-rec-fsharp-dotnet7-bug-demo>.$Program.main@() in /workspaces/function-rec-fsharp-dotnet7-bug-demo/Program.fs:line 29
*)

The sample repo includes a devcontainer set up for simple testing - just dotnet run.

Related information

Provide any related information (optional):

• Ubuntu 21.10
• .NET SDK 7.0.100-preview.3.22179.4

I suspect this issue will be eventually closed in favor of a runtime issue, but know too little to report there appropriately.

[KevinRansom]

@vzarytovskii , we probably need to evaluate the debugging experience more deeply, before we do that. This change, makes no difference in the debugging as far as I can tell.

[En3Tho]

I’m able to reproduce it on 6.0.300-preview.22154.4 too. There is an IL diff between stable and preview versions: stable (6.0.104) : both functions have effectively identical il https://www.diffchecker.com/Av9sdqwx

preview: IL of “function” is very different. It is on the left.

It seems that compiler is now trying to make “f” an object instead of function.

So it tries to get “f” from field which is somehow null and fails on “f tail” call. You can check that by printfn ReferenceEquals(f, null) before the call. Funnily, removing type annotation from “let rec f : List<int> -> List<int>” to just “let rec f” fixes an issue.

Side note: I wonder why compiler doesn’t emit a warning when rec function results in closure instead of a while loop. I would really like to see a warning. Some things are really better expressed as rec’s but it’s always a manual check that assures, not compiler.

https://www.diffchecker.com/eAcGUo5B

Oh, @vzarytovskii was faster than me 😃