Cannot receive editor support for regions inside compiler directives enabled by traits

[bioball]

Swift version

swift-driver version: 1.148.6 Apple Swift version 6.3.2 (swiftlang-6.3.2.1.108 clang-2100.1.1.101)

Platform

arm64-apple-macosx26.0

Editor

Visual Studio Code

Description

I'm working on code that is enabled by a trait, and cannot get sourcekit-lsp to provide editor support for code that is inside regions that are conditionally included via traits.

Here is my locally set .sourcekit-lsp/config.json:

{
  "swiftPM": {
    "traits": ["libpkl", "libpkl_shared"]
  }
}

Here's a reproducer branch; focus on file LibPklClient.swift:

https://github.com/bioball/pkl-swift/tree/sourcekit-bug

Trying to compile with swift build --traits libpkl,libpkl_shared produces error:

Building for debugging...
/Users/danielchao/code/apple/pkl-swift/Sources/PklSwift/LibPkl/LibPklClient.swift:69:14: error: value of type 'LibPklClient' has no member 'foobar'
 67 |
 68 |         // nonsense; `foobar` doesn't exist on `self`
 69 |         self.foobar = "hi"
    |              `- error: value of type 'LibPklClient' has no member 'foobar'
 70 |
 71 |         let context = ResponseContext(continuation: continuation)
[6/9] Compiling PklSwift LibPklClient.swift

However, VSCodes doesn't highlight this error. Also, other basic things don't work, e.g. go-to definition.

Steps to Reproduce

1. Clone this branch https://github.com/bioball/pkl-swift/tree/sourcekit-bug
2. Add this file to .sourcekit-lsp/config.json:

   {
     "swiftPM": {
       "traits": ["libpkl", "libpkl_shared"]
     }
   }
   

3. Navigate to LibPklClient.swift inside Visual Studio Code, and find that the editor isn't showing any error diagnostics, and other things don't work either (e.g. go-to-definition)

NOTE: running swift build produces errors as expected:

Building for debugging...
/Users/danielchao/code/apple/pkl-swift/Sources/PklSwift/LibPkl/LibPklClient.swift:69:14: error: value of type 'LibPklClient' has no member 'foobar'
 67 |
 68 |         // nonsense; `foobar` doesn't exist on `self`
 69 |         self.foobar = "hi"
    |              `- error: value of type 'LibPklClient' has no member 'foobar'
 70 |
 71 |         let context = ResponseContext(continuation: continuation)
[6/9] Compiling PklSwift LibPklClient.swift

Logging

No response