Transparent Compiler returns no completions after a dangling dot

[MangelMaxime]

Hello,
While working on a project of mine I discovered some strange behavior in the intellisense results.

After some investigation, here is what I found:

Investigation# Transparent Compiler returns no completions after a dangling dot

With the transparent compiler enabled, GetDeclarationListInfo returns an empty list for Questions. when Questions is a module declared in an earlier file of the project and the namespace holding it declares no type of its own.

The default background compiler returns the expected list for the same input.

Reproduces on FSharp.Compiler.Service 43.9.300, 43.10.103, 43.12.400, and 43.1001.125 (the build shipped in FsAutoComplete 0.83.0).

Run

./run.sh

transparentCompiler=false  2 completions [|"singleton"; "count"|]
transparentCompiler=true   0 completions [||]

Pass -p:FcsVersion=43.9.300 to run.sh to use a different FCS build.

Input

Library/Questions.fs

namespace Demo

module Questions =

    let singleton = 1
    let count = 2

Library/Use.fs

namespace Demo

module Use =

    let f () =
        Questions.
        ()

Use.fs has one parse error, FS0599: Missing qualification after '.'. The harness asks for completions at the end of the Questions. line.

Observations

Editing Library/Questions.fs and rerunning gives:

Questions.fs shape	default	transparent
namespace holds only the module (as checked in)	2	0
a type added to the namespace, after the module	2	2
a type added to the namespace, before the module	2	2
a type added inside the module	3	0

Writing Demo.Questions. in Use.fs instead of Questions. changes nothing.

These are correct under both compilers, so the failure is not general to the dangling dot:

• a module declared in the same file as the dot
• an FSharp.Core module, such as String.
• a .NET type, such as System.Console.
• an instance member, such as s. on a string

Adding an unrelated statement to f does not help. Adding one that mentions Questions does:

    let f () =
        Questions.
        Questions.singleton |> ignore
        ()

With that line present the transparent compiler returns the 2 expected completions.

Effect in an editor

FsAutoComplete passes --use-fcs-transparent-compiler when the VSCode setting FSharp.fcs.transparentCompiler.enabled is true.

Typing a dot after such a module then offers nothing.

Layout

• Library/ - the project being analysed. Its DumpFscArgs target writes the compiler arguments of a design-time build to Library/obj/args.txt.
• Harness/ - console app that calls ParseAndCheckFileInProject and GetDeclarationListInfo against Library, once per compiler.
• run.sh - runs the target, then the harness.

fcs-transparent-completion-repro.zip

Related information

Provide any related information (optional):

• Operating system: Omarchy (Arch Linux based), kernel 7.2.3-arch1-3, x86_64
• .NET Runtime kind: .NET (Core)
  • SDK 10.0.102, harness and repro project both target net8.0 and run on .NET 8.0.19
  • Installed runtimes: 6.0.36, 8.0.19, 9.0.11, 10.0.0, 10.0.1, 10.0.2, 10.0.8
• Editing Tools: VSCode 1.137.0 with Ionide-fsharp 7.31.1 (FsAutoComplete 0.83.0), setting FSharp.fcs.transparentCompiler.enabled set to true
• FSharp.Compiler.Service: reproduces on 43.9.300, 43.10.103, 43.12.400, and 43.1001.125 (the build inside FsAutoComplete 0.83.0)