Unexpected warning '[FS3511] This state machine is not statically compilable'

[BoundedChenn31]

We have a number of problems where tasks are not being statically compiled.

• One is below.
• Another is FS3511 warning when using for...in in task CE with .NET SDK 6.0.400 in Release builds #13657
• For top-level tasks, see task defined as top-level static value doesn't get statically compiled #12038

See also demetrixbio/Plough.WebApi#5 as an example where a library hit this.

---

Repro steps

I had this problem in more complicated code, however it boils down to something like this:

type Foo = { X: int option }

type BigRecord =
    {
        a1: string
        a2: string
        a3: string
        a4: string
        a5: string
        a6: string
        a7: string
        a8: string
        a9: string
        a10: string
        a11: string
        a12: string
        a13: string
        a14: string
        a15: string
        a16: string
        a17: string
        a18: string
        a19: string
        a20: string
        a21: string
        a22: string
        a23: string
        a24: string
        a25: string
        a26: string
        a27: string
        a28: string
        a29: string
        a30: string
        a31: string
        a32: string
        a33: string
        a34: string
        a35: string
        a36: string // no warning if at least one field removed

        a37Optional: string option
    }

let testStateMachine (bigRecord: BigRecord) =
    task {
        match Some 5 with // no warn if this match removed and only inner one kept
        | Some _ ->
            match Unchecked.defaultof<Foo>.X with // no warning if replaced with `match Some 5 with`
            | Some _ ->
                let d = { bigRecord with a37Optional = None } // no warning if d renamed as _ or ignore function used
                ()
            | None -> ()
        | _ -> ()
    }

printfn "Hello from F#"

Then compile it in release mode.

Original code doesn't have Unchecked.defaultof and gets Foo from another function but I wasn't able to create minimal example without it. I guess, it doesn't really matter and root cause is the same.

Expected behavior

No warning or better explanation of the problem and how to fix it.

Actual behavior

When compiling in Release mode:

dotnet build -c Release

Program.fs(46, 5): [FS3511] This state machine is not statically compilable. A resumable code invocation at '(46,4--46,8)' could not be reduced. An alternative dynamic implementation will be used, which may be slower. Consider adjusting your code to ensure this state machine is statically compilable, or else suppress this warning.

Known workarounds

In original code I had to create another function that works with d and inline usage let d = { bigRecord with a37Optional = None } variable at line 51:

...
match Unchecked.defaultof<Foo>.X with
| Some _ ->
    do! testStateMachineInner { bigRecord with a37Optional = None }
| None -> ()
...

Related information

.NET SDK (reflecting any global.json):
Version: 6.0.201
Commit: ef40e6aa06

Runtime Environment:
OS Name: Windows
OS Version: 10.0.19044
OS Platform: Windows
RID: win10-x64
Base Path: C:\Program Files\dotnet\sdk\6.0.201\

Host (useful for support):
Version: 6.0.3
Commit: c24d9a9c91

[cmeeren]

I experience something similar. Here is my repro. Let me know if I should post a separate issue for this.

type T = {
  A: string
  B: string
  C: int64
}

let f () =
  task {
    // No warning if removing outer or inner match
    match Error "" with
    | Error _ -> ()
    | Ok _ ->
        match Error "" with
        | Error _ -> return failwith ""  // No warning if replacing with () as above
        | Ok _ ->
            match Some (Unchecked.defaultof<T>, Unchecked.defaultof<unit>) with  // No warning if using literal () or removing second tuple element
            | None -> return failwith ""
            | Some (info, _a) ->  // No warning if renaming '_a' to '_'
                printfn $"{info.A}"  // No warning if removing
                printfn $"{info.B}"  // No warning if removing
                printfn $"{info.C}"  // No warning if removing
                printfn $"{2 + 2}"  // No warning if removing or replacing with constant string
                return failwith ""  // No warning if removing or replacing with 'return ()' or '()'
  }

In reducing this repro, it seemed extremely arbitrary to me what caused the problem and what doesn't. For the original code (which is a Giraffe HttpHandler for serving file downloads), I am left with no apparent recourse other than to ignore FS3511 for the whole project, and live with whatever the performance impacts are.

[abelbraaksma]

Probably related with #12038.

[cmeeren]

Just for the record:

In that issue, @dsyme said:

It's not a super-critical show-stopping bug because

1. the task still runs correctly
2. it's rare to define tasks like this in project code (however it may happen a lot in notebooks)
3. tasks at the top-level like this are never perf-critical

Note that points 2 and 3 don't apply here.

[abelbraaksma]

@kerams, can you make the repro reproducible, please? I.e., by adding stubs for the functions that you omitted? That will make it easier to analyze what's going on.