Tooling implies that `_` in member body refers to the instance identifier when it is actually a wildcard pattern

[brianrourkeboll]

When an instance member uses a single underscore _ as its "this" or instance identifier (fsharp/fslang-suggestions#333), the F# tooling implies that any _ pattern used in the member body refers to the instance, even though it does not.

Repro steps

type T () =
    member _.M () =
        fun _ -> ()
        //  ↑ Shows unexpected val _:T on hover.

    member _.N () =
        let _ = () in ()
        //  ↑ Shows unexpected val _:T on hover.

    member this.O () =
        let _ = () in ()
        //  ↑ Shows nothing on hover, as expected.

Expected behavior

Any _ patterns in a member body whose instance identifier is also _ should nonetheless be treated by the tooling like wildcard patterns are in general, and nothing should be shown on hover.

expected.mp4

Actual behavior

Any _ patterns in a member body whose instance identifier is also _ are treated by the tooling as though they refer to the instance identifier.

unexpected.mp4

Known workarounds

N/A.

Related information

Applies to VS and VS Code, .NET 8 and 9 (and probably earlier?).

[T-Gro]

🤖 Copilot implementation hints

Fix

One-line change in src/Compiler/Checking/Expressions/CheckExpressions.fs in MakeAndPublishVal (~line 1500).

The shouldNotifySink helper suppresses sink notifications for member-this vals named "__" but not "_". The _ self val gets published to the name env, causing tooltips on wildcard _ patterns to show val _: T.

Change:

vspec.IsMemberThisVal && vspec.LogicalName = "__"

to:

vspec.IsMemberThisVal && (vspec.LogicalName = "__" || vspec.LogicalName = "_")

Instructions

• Use TDD: write a tooltip test for member _.M () = fun _ -> () asserting no symbol is reported on the wildcard _ in the lambda.
• Run: dotnet test tests/FSharp.Compiler.Service.Tests/FSharp.Compiler.Service.Tests.fsproj --filter "Tooltip" -c Debug
• After code change, invoke the expert-review skill to review your work.
• Minimal surgical fix. No bloated comments.
• Run dotnet fantomas . on changed files.