Issue description

A specific scenario is broken by enabling nullness checking, manifesting as FS0001.

Choose one or more from the following categories of impact

• Unexpected nullness warning (false positive in nullness checking, code uses --checknulls and langversion:preview).
• Missing nullness warning in a case which can produce nulls (false negative, code uses --checknulls and langversion:preview).
• Breaking change related to older null constructs in code not using the checknulls switch.
• Breaking change related to generic code and explicit type constraints (null, not null).
• Type inference issue (i.e. code worked without type annotations before, and applying the --checknulls enforces type annotations).
• C#/F# interop issue related to nullness metadata.
• Other (none of the categories above apply).

Operating System

Windows (Default)

What .NET runtime/SDK kind are you seeing the issue on

.NET SDK (.NET Core, .NET 5+)

.NET Runtime/SDK version

.NET 9

Reproducible code snippet and actual behavior

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net9.0</TargetFramework>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="Program.fs" />
  </ItemGroup>

  <ItemGroup>
    <PackageReference Include="FSharpPlus" Version="1.6.1" />
  </ItemGroup>

</Project>

open FSharpPlus

let parseInt () : Result<int, string> =
    Ok 42

type SomeRecordWrapper = {
    Value: int
}

let repro () =
    let result =
        monad {
            let! parsed = parseInt ()
            return {
                Value = parsed
            }
        }

    match result with
    | Ok _ ->
        ()
    | Error _ ->
        ()

With Nullable enabled, this results in:

0>Program.fs(20,7): Error FS0001 : '.Delay' does not support the type 'Result<SomeRecordWrapper,'a>', because the latter lacks the required (real or built-in) member 'Delay'
0>Program.fs(19,11): Warning FS0025 : Incomplete pattern matches on this expression.

With Nullable disabled, the code compiles fine.

Possible workarounds

None that I know of.