Semantic Highlighting regression: optional parameter still highlights as namespace (GetPartialLongNameEx missing fix)

[yTex10]

Summary

Despite the fix for optional parameter identifier extraction (in QuickParse.GetCompleteIdentifierIsland) and release in 10.0.200, semantic highlighting in Visual Studio (18.5+, FSharp 10.0.202) still incorrectly colors functions with optional parameters as if the entire function is a namespace (see screenshot below).

---

---

Repro steps

1. Open a file with this code:

type TestType() =
    member x.TestOptional(?optionalParameter) = optionalParameter

2. Observe that TestOptional function and its contents get highlighted as a namespace or type, not as a method/member and parameter.

Expected behavior

Only optionalParameter should be highlighted as a parameter, and the function as a member.

Actual behavior

The whole function is colored as if it were a namespace. Note that using just a ? as the parameter name (e.g. x.TestOptional(?)) will highlight the ? as an identifier(and yes, that compiles), but the highlighting error is going to occur after you add the parameter name(identifier) after the ? symbol.

Cause

• The fix was applied to QuickParse.GetCompleteIdentifierIsland, but not to QuickParse.GetPartialLongNameEx, which is used for semantic classification and highlighting in Visual Studio.
• The test suite only covers GetCompleteIdentifierIsland, so tests falsely pass.
• GetPartialLongNameEx lacks recognition of the syntax ?identifier, so everything after ? is not recognized as a proper parameter and falls back to treating the context as a namespace.

Known workaround

None yet.

Related info

• F# VS extension 18.5.0, FSharp.Compiler.Service 10.0.202
• Windows 11, .NET 10 SDK
• Original Issue
• Original Fix, applied for .NET 10.0.200 SDK
• QuickParse.fs Current code
• Related Visual Studio incident

Recommendation

Apply the same logic for optional parameters (? handling) in GetPartialLongNameEx, just as in GetCompleteIdentifierIsland, and add/expand test coverage for this case.

---

Reported by Copilot user after upgrade to 10.0.202 & VS 18.5.

CC: @fsharp/vs-editor-tools @fsharp/compiler-contributors