Signature file generation chops off leading `FSharp.` prefix for externally-defined code

[baronfel]

Given a member or function that takes a parameter or returns a value of an externally-defined type that begins with FSharp, for example FSharp.Control.AsyncSeq<'t> from the FSharp.Control.AsyncSeq nuget package, generating a signature for that member or function

Repro steps

Provide the steps required to reproduce the problem:

Given this project file:

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

  <PropertyGroup>
    <TargetFramework>net5.0</TargetFramework>
    <OtherFlags>--allsigs</OtherFlags>
  </PropertyGroup>

  <ItemGroup>
    <Compile Include="FSharpPrefix.fs" />
    <PackageReference Include="FSharp.Control.AsyncSeq" Version="3.1.0" />
  </ItemGroup>

</Project>

and this source file:

namespace FSharp.MyStuff

module Library =
    open FSharp.Control

    let batch size (s: AsyncSeq<'t>) =
        AsyncSeq.bufferByCount 12 s

run dotnet build to generate the FSharpPrefix.fsi signature file, which looks like this:

namespace FSharp.MyStuff
  module Library = begin
    val batch : size:'a -> s:Control.AsyncSeq<'t> -> Control.AsyncSeq<'t []>
  end

If this file is added to the project file above the source file, the build fails with the following error:

/Users/chethusk/oss/scratch/FSharpPrefix.fsi(3,38): error FS0039: The type 'AsyncSeq' is not defined in 'Microsoft.FSharp.Control'. Maybe you want one of the following:   Async   AsyncResult   Async`1   AsyncResult`1   AsyncReturn [/Users/chethusk/oss/scratch/scratch.fsproj]

Expected behavior

The signature should be generated with fully-qualified names for types in the FSharp namespace but that are externally-defined

Actual behavior

The leading FSharp is stripped, possibly due to some heuristic?

Known workarounds

Manually edit the signature after generation (which can be rough if --allsigs is enabled in the project file).

[weslenng]

I'm trying to solve this issue and started going through the compiler call chain. I've found where the declarations are type-checked and (later) written to a file

The problem is that FSharp.Control is open by default so fsc omit the prefix, right?
Do you guys have any suggestions on where I can start?

[dsyme]

This is actually quite subtle and might need my attention I think.

The sequence is this

1. "FSharp" and "FSharpMyStuff" get opened by default (added to DisplayEnv.openTopPathsSorted) because we're in the namespace "FSharp.FSharpMyStuff". This "open" is not however the same as the kind of "open" you get with "open FSharp" - it is a limited open that doesn't bring sub-namespaces into scope, e.g. Control.AsyncSeq2 doesn't come into scope. This difference is the root cause of the problem

2. The presence of "FSharp" in denv.openTopPathsSorted causes trimPathByDisplayEnv to trim "FSharp" from "FSharp.Control.AsyncSeq"

3. "Control.AsyncSeq" no longer resolves in the signature file because the implicit open of "FSharp" is not sufficient per above.

The "openTopPathsSorted" in "DisplayEnv" really need to carry a flag indicating when kind of "open" they relate to, and the logic in trimPathByDisplayEnv needs to be adjusted for this.