Mutually recursive computation expression raises NullReferenceException under some conditions

[aqtq314]

Given the following function definition,

let x () =
    let rec a = seq {
        yield 0
        yield! b () }
    and b () = seq {
        yield 1
        yield! a }
    do Seq.take 10 a |> Seq.iter (printfn "%d")

Invoking x () will print a 0 then raises NullReferenceException. Putting the function body into a type constructor generates a similar behavior when creating an instance of it:

type A () =
    do  let rec a = seq {
            yield 0
            yield! b () }
        and b () = seq {
            yield 1
            yield! a }
        do Seq.take 10 a |> Seq.iter (printfn "%d")

One of the workarounds would be inlining the do-binding in the above type definition, like below:

type A () =
    let rec a = seq {
        yield 0
        yield! b () }
    and b () = seq {
        yield 1
        yield! a }
    do Seq.take 10 a |> Seq.iter (printfn "%d")

Another workaround is to modify a so that it becomes a function that takes in a parameter:

type A () =
    do  let rec a () = seq {
// ...

Environment:

• VS2017
• .NET Framework 4.5 or upper
• FSharp.Core 4.4.0 or upper

This doesn't happen in VS2015 or on www.ideone.com.

[cartermp]

This looks like a dupe of #3743 - @aqtq314 are you using VS 2017 15.4?

[aqtq314]

I'm on VS Community 2017 15.2 (26430.16), and I cannot reproduce #3743 on my side.
But it does look like the underlying cause is similar.

[aqtq314]

Oh and release mode doesn't help in this one.

[aqtq314]

clicked the wrong button D:

[cartermp]

I can reproduce this on 15.4.1. Interesting that you can see this on 15.2. @dsyme's PR (#3536) which seems to have caused #3734 was merged long after 15.2 shipped. @dsyme any ideas?

[dsyme]

This is a separate bug. It's odd as we've not had any bugs in closure construction for a long time.

Anyway, it' not a regression but obviously needs to be fixed