Task code requries additional type annotations (was : Runtime cast failure using tasks in F# 6.0 )

[Lanayx]

Please provide a succinct description of the issue.

Repro steps

open System.Threading.Tasks

let myFunction (f, i) =
    task {
        do! f ""
        return ()
    }

[<EntryPoint>]
let main argv =
    let myTuple = (fun s -> Task.FromResult()), 1
    (myFunction myTuple).Wait()
    
    0

Expected behavior

The programs runs to end successfully. This code worked with Fsharp 5.0.1 and TaskBuilder.fs

Actual behavior

With Fsharp.Core 6.0 it fails in runtime with error

Unhandled exception. System.InvalidCastException: Unable to cast object of type 'System.Tuple`2[Microsoft.FSharp.Core.FSharpFunc`2[System.String,System.Threading.Tasks.Task`1[Microsoft.FSharp.Core.Unit]],System.Int32]' to type 'S
ystem.Tuple`2[Microsoft.FSharp.Core.FSharpFunc`2[System.String,System.Threading.Tasks.Task`1[Microsoft.FSharp.Core.Unit]],System.Object]'.

Known workarounds

It works when f or i parameter has type annotation

Related information

Provide any related information (optional):

• Windows 10
• Microsoft (R) Build Engine version 17.0.0-preview-21460-01+8f208e609 for .NET
• Jetbrains Rider

[dsyme]

Thanks for this bug report. I've changed the title.

The underlying problem here is the use of SRTP for "task-like" constructs in F# 6.0, which is hitting problems associated right back to the mis-inclusion of #1650 into F#. I will find a suitable fix for this and we will also be adjuting SRTP to fully remove #1650 in F# vNext, one way or another.

It will take a while for that fix to come through the works. In the meantime you should indeed add type annotations to places where the types of tasks being executed by do! or let! is not simply determined. For example

let myFunction (f: string -> Task<_>, i) =
    task {
        do! f ""
        return ()
    }

[dsyme]

Fixed by #12195