Fix preferreduilang switch leaking into fsi.CommandLineArgs

[Copilot]

Implementation Plan for preferreduilang Switch in FSI

Phase 1: Core Implementation

• Add preferreduilang option to FSI option parsing in src/Compiler/Interactive/fsi.fs
  • Modified script.fsx handler to filter out --preferreduilang and /preferreduilang
  • Parse culture string and set tcConfigB.preferredUiLang and Thread.CurrentThread.CurrentUICulture
  • Ensure option is consumed before ReportUserCommandLineArgs
  • Handle parsing errors gracefully - validate culture before setting tcConfigB
• Tested manually - switch is consumed from CommandLineArgs in all positions

Phase 2: Testing

• Add CLI regression tests for argument separation
  • Test: switch before script → consumed from CommandLineArgs
  • Test: switch after script → consumed from CommandLineArgs
  • Test: localization verification (culture is set)
  • Test: both --preferreduilang and /preferreduilang forms work
  • Test: invalid culture handled gracefully ✓ Fixed
• All 6 tests passing

Phase 3: Validation

• Run formatting with fantomas
• Run code review
• Fixed test failure for invalid culture handling
• Added release notes

Original prompt

Implement proper handling of the preferreduilang switch in FSI and add regression tests.

Scope:

1. Consume and apply --preferreduilang//preferreduilang in FSI option parsing so it does not surface in fsi.CommandLineArgs, and set localization accordingly.

   • Add a recognized option in src/Compiler/Interactive/fsi.fs (option table near OptionGeneral("script.fsx arg1 arg2 ...")).
   • Support both /preferreduilang:<culture> and --preferreduilang:<culture>.
   • On parse: set tcConfigB.lcid and Thread.CurrentThread.CurrentUICulture (and possibly CurrentCulture) to the parsed culture; if parsing fails, warn and ignore.
   • Ensure this happens before ReportUserCommandLineArgs so the switch is consumed and not forwarded to scripts.

2. Add tests for argument separation and localization.

   • CLI regression tests (new folder tests/fsharp/core/interactive-args/ or extend existing tests/fsharpqa/Source/InteractiveSession/Misc/CommandLineArgs01*.fs):
     a) dotnet fsi --preferreduilang:en-US --exec script.fsx a → fsi.CommandLineArgs = ["script.fsx"; "a"] (switch consumed).
     b) dotnet fsi script.fsx --preferreduilang:en-US a → fsi.CommandLineArgs = ["script.fsx"; "a"] (even after script).
     c) Localization: dotnet fsi --preferreduilang:es-ES --exec script.fsx where the script triggers a warning/error; assert the culture is set (CurrentUICulture.Name starts with es-ES) and the message is localized or falls back if resources are missing. Consider a second locale (e.g., fr-FR).

3. MSBuild-path regression test.

   • Add a sample project under tests/projects/FSI/PreferreduilangRepro/ that runs a script printing fsi.CommandLineArgs and CurrentUICulture.Name.
   • Invoke via dotnet msbuild (or existing VS integration harness) so the ToolTask-injected /preferreduilang is present.
   • Assert the switch is not in CommandLineArgs and the culture is applied.

4. Notes/guardrails:

   • Do not leak preferreduilang into explicitArgs/CommandLineArgs regardless of position relative to the script token.
   • If culture parsing fails, warn but do not treat it as a script arg.
   • Keep behavior consistent between CLI and MSBuild paths.

References:

• Option parsing & script arg handling: src/Compiler/Interactive/fsi.fs (option table around ~1053–1108; final ReportUserCommandLineArgs around ~1208–1222).
• Public contract for fsi.CommandLineArgs: src/Compiler/Interactive/fsi.fsi (~397–412).
• MSBuild host that may inject /preferreduilang: src/FSharp.Build/Fsi.fs (ToolTask; captured args around ~421–424).

This pull request was created as a result of the following prompt from Copilot chat.

Implement proper handling of the preferreduilang switch in FSI and add regression tests.

Scope:

1. Consume and apply --preferreduilang//preferreduilang in FSI option parsing so it does not surface in fsi.CommandLineArgs, and set localization accordingly.

   • Add a recognized option in src/Compiler/Interactive/fsi.fs (option table near OptionGeneral("script.fsx arg1 arg2 ...")).
   • Support both /preferreduilang:<culture> and --preferreduilang:<culture>.
   • On parse: set tcConfigB.lcid and Thread.CurrentThread.CurrentUICulture (and possibly CurrentCulture) to the parsed culture; if parsing fails, warn and ignore.
   • Ensure this happens before ReportUserCommandLineArgs so the switch is consumed and not forwarded to scripts.

2. Add tests for argument separation and localization.

   • CLI regression tests (new folder tests/fsharp/core/interactive-args/ or extend existing tests/fsharpqa/Source/InteractiveSession/Misc/CommandLineArgs01*.fs):
     a) dotnet fsi --preferreduilang:en-US --exec script.fsx a → fsi.CommandLineArgs = ["script.fsx"; "a"] (switch consumed).
     b) dotnet fsi script.fsx --preferreduilang:en-US a → fsi.CommandLineArgs = ["script.fsx"; "a"] (even after script).
     c) Localization: dotnet fsi --preferreduilang:es-ES --exec script.fsx where the script triggers a warning/error; assert the culture is set (CurrentUICulture.Name starts with es-ES) and the message is localized or falls back if resources are missing. Consider a second locale (e.g., fr-FR).

3. MSBuild-path regression test.

   • Add a sample project under tests/projects/FSI/PreferreduilangRepro/ that runs a script printing fsi.CommandLineArgs and CurrentUICulture.Name.
   • Invoke via dotnet msbuild (or existing VS integration harness) so the ToolTask-injected /preferreduilang is present.
   • Assert the switch is not in CommandLineArgs and the culture is applied.

4. Notes/guardrails:

   • Do not leak preferreduilang into explicitArgs/CommandLineArgs regardless of position relative to the script token.
   • If culture parsing fails, warn but do not treat it as a script arg.
   • Keep behavior consistent between CLI and MSBuild paths.

References:

• Option parsing & script arg handling: src/Compiler/Interactive/fsi.fs (option table around ~1053–1108; final ReportUserCommandLineArgs around ~1208–1222).
• Public contract for fsi.CommandLineArgs: src/Compiler/Interactive/fsi.fsi (~397–412).
• MSBuild host that may inject /preferreduilang: src/FSharp.Build/Fsi.fs (ToolTask; captured args around ~421–424).

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[T-Gro]

@copilot :

Failed Scripting.PreferredUiLang tests.invalid culture in preferreduilang is ignored gracefully [22 ms]
Error Message:
System.Globalization.CultureNotFoundException : Culture is not supported.
Parameter name: name
invalid-culture-xyz is an invalid culture identifier.
Stack Trace:
at System.Globalization.CultureInfo..ctor(String name, Boolean useUserOverride)
at FSharp.Compiler.Interactive.Shell.FsiEvaluationSession..ctor(FsiEvaluationSessionHostConfig fsi, String[] argv, TextReader inReader, TextWriter outWriter, TextWriter errorWriter, Boolean fsiCollectible, FSharpOption`1 legacyReferenceResolver) in D:\a_work\1\s\src\Compiler\Interactive\fsi.fs:line 4663
at FSharp.Test.CompilerAssert.RunScriptWithOptionsAndReturnResult(String[] options, String source) in D:\a_work\1\s\tests\FSharp.Test.Utilities\CompilerAssert.fs:line 987
at Scripting.PreferredUiLang tests.invalid culture in preferreduilang is ignored gracefully()

[Copilot]

@copilot :

Failed Scripting.PreferredUiLang tests.invalid culture in preferreduilang is ignored gracefully [22 ms]
...

Fixed in 328b7b3. The issue was that tcConfigB.preferredUiLang was being set before validating the culture, causing a later throw when the session tried to create a CultureInfo from the invalid value. Now the culture is validated first by creating the CultureInfo object, and only if successful are both tcConfigB.preferredUiLang and CurrentUICulture set. All tests now pass.

[github-actions]

❗ Release notes required

---

✅ Found changes and release notes in following paths:

Change path	Release notes path	Description
src/Compiler	docs/release-notes/.FSharp.Compiler.Service/10.0.200.md

[T-Gro]

@copilot :

add release notes to docs/release-notes/.FSharp.Compiler.Service/10.0.200.md

[Copilot]

@copilot :

add release notes to docs/release-notes/.FSharp.Compiler.Service/10.0.200.md

Added in 6e92280. Release notes entry added to the Fixed section documenting the fix for --preferreduilang switch leaking into fsi.CommandLineArgs.