Conformant libc++21 formatter refactor (boundary widening) [WIP] by benhillis · Pull Request #41072 · microsoft/WSL

benhillis · 2026-07-13T23:05:00Z

Draft / WIP. Follow-up to #41071 (which ships the minimal #ifdef WIN32 guard as the shipping fix).

This branch explores the fully conformant alternative: instead of keeping the formatter<char*, wchar_t> / formatter<std::string, wchar_t> specializations (deleted in C++20/23 [format.formatter.spec] and shipped as deleted specializations in libc++ 20+), it removes them and widens narrow arguments explicitly.

Approach

The key insight is a single boundary helper rather than per-call-site churn:

wsl::shared::string::LocalizationArg() (stringshared.h) widens narrow string args (const char*, char[N], std::string, std::string_view) to wchar_t on Windows, and is an identity forward on Linux (where messages are already narrow).
The Localization generator (generateLocalizationHeader.ps1) routes every Message*() argument through it, so std::vformat(message, ARGS(LocalizationArg(arg)...)).

This fixes all product code (common, wslc, wslcsession, service.exe) through one generator change.

Remaining work

Direct std::format(L"...", narrowArg) sites that do not go through Localization (primarily test/windows unit tests) still rely on the removed specializations and need conversion.
Decide whether the conformant path is worth the test-side churn versus the shipped guard in Bump Linux SDK to 1.23.0 and guard char->wide formatters to Windows #41071.

Copilot

Pull request overview

This PR explores a C++20/23-conformant approach to formatting localized messages in the presence of libc++21’s deleted cross-encoding std::formatter specializations by removing the repo’s narrow→wide formatters and instead widening narrow arguments at a single boundary.

Changes:

Updated the localization header generator to route every Message*() formatting argument through wsl::shared::string::LocalizationArg().
Introduced LocalizationArg() and removed the repo’s std::formatter<char*/*, wchar_t>-style specializations, plus updated several call sites to explicitly widen narrow strings where needed.
Bumped Microsoft.WSL.LinuxSdk from 1.20.0 to 1.23.0.

Reviewed changes

Copilot reviewed 30 out of 30 changed files in this pull request and generated 7 comments.

Show a summary per file

File	Description
tools/generateLocalizationHeader.ps1	Routes generated `Message*()` formatting arguments through `LocalizationArg()` before `std::vformat`.
src/shared/inc/stringshared.h	Adds `LocalizationArg()` boundary helper; removes narrow→wide formatter specializations; updates `source_location` wide formatting.
src/windows/common/ExecutionContext.h	Normalizes user-error message conversion via `ToUserErrorMessage()` instead of formatting-based coercion.
src/windows/wslcsession/WSLCVolumes.cpp	Explicitly widens narrow args passed into localized warnings/errors.
src/windows/wslcsession/WSLCVirtualMachine.cpp	Explicitly widens narrow port-string argument for localized error.
src/windows/wslcsession/WSLCVhdVolume.cpp	Explicitly widens several narrow args in localized errors/warnings.
src/windows/wslcsession/WSLCSession.cpp	Widens narrow identifiers/names passed into localized user errors.
src/windows/wslcsession/WSLCGuestVolume.cpp	Widens narrow option strings used in localized error paths.
src/windows/wslcsession/WSLCContainer.cpp	Widens various narrow IDs/names used in localized user errors/warnings.
src/windows/wslcsession/OptionParser.cpp	Widens option key/value strings for localized user errors.
src/windows/wslc/tasks/VolumeTasks.cpp	Widens volume names printed via localized messages.
src/windows/wslc/tasks/NetworkTasks.cpp	Widens network names printed via localized messages.
src/windows/wslc/tasks/ImageTasks.cpp	Widens image IDs/refs for localized output; widens ID printing in list path.
src/windows/wslc/tasks/ContainerTasks.cpp	Widens container IDs printed via localized messages.
src/windows/wslc/services/SessionService.cpp	Widens command name passed into localized launch-failure error.
src/windows/wslc/services/ImageService.cpp	Widens image reference passed into localized tag-format error.
src/windows/wslc/services/ImageProgressCallback.cpp	Fixes direct wide `std::format` uses by widening `LPCSTR` inputs.
src/windows/wslc/commands/VersionCommand.cpp	Fixes wide `std::format` call by widening `WSL_PACKAGE_VERSION`.
src/windows/service/exe/WslCoreVm.cpp	Widens enum-to-string values before passing to localized warning.
src/windows/service/exe/LxssUserSession.cpp	Widens JSON parse error text; adjusts error-string handling for localized warning.
src/windows/common/wslutil.cpp	Widens Windows version / IP / image strings passed into localized messages.
src/windows/common/WslCoreConfig.cpp	Widens config key/value strings passed into localized warnings.
src/windows/common/WslClient.cpp	Widens strerror/version/build-info narrow inputs used in localized output.
src/windows/common/string.cpp	Ensures hex-string invalid input is widened appropriately for localized error formatting.
src/windows/common/notifications.cpp	Widens `binaryName` passed into localized toast content.
src/windows/common/ConsommeNetworking.cpp	Widens host address string in wide `std::format` call.
src/shared/inc/JsonUtils.h	Widens exception message for localized invalid-JSON user error on Windows.
src/shared/configfile/configfile.h	Widens enum-parse warning arguments before calling localization.
src/shared/configfile/configfile.cpp	Widens multiple config warning arguments before calling localization.
packages.config	Updates Linux SDK NuGet version to 1.23.0 (and introduces a leading BOM).

+        EMIT_USER_WARNING(Localization::MessageConfigInvalidBoolean(
+            wsl::shared::string::MultiByteToWide(value), wsl::shared::string::MultiByteToWide(name), filePath, fileLine));
        return false;


+        EMIT_USER_WARNING(Localization::MessageConfigInvalidInteger(
+            wsl::shared::string::MultiByteToWide(value), wsl::shared::string::MultiByteToWide(name), filePath, fileLine));
        return false;


+        EMIT_USER_WARNING(wsl::shared::Localization::MessageInvalidNumberString(
+            wsl::shared::string::MultiByteToWide(value), wsl::shared::string::MultiByteToWide(name), filePath, fileLine));
        return false;


+        EMIT_USER_WARNING(Localization::MessageConfigKeyDuplicated(
+            wsl::shared::string::MultiByteToWide(name),
+            fileName,
+            line,
+            wsl::shared::string::MultiByteToWide(m_parseResult->first),
+            fileName,
+            m_parseResult->second));


+        EMIT_USER_WARNING(Localization::MessageConfigUnknownKey(wsl::shared::string::MultiByteToWide(keyName), filePath, fileLine));
        return;


+            EMIT_USER_WARNING(wsl::shared::Localization::MessageConfigInvalidEnum(
+                wsl::shared::string::MultiByteToWide(value),
+                wsl::shared::string::MultiByteToWide(name),
+                fileName,
+                line,
+                wsl::shared::string::MultiByteToWide(validValues.str())));


+    if constexpr (std::is_same_v<std::remove_cvref_t<T>, std::string> || std::is_same_v<std::remove_cvref_t<T>, std::string_view>)
+    {
+        return MultiByteToWide(std::string{value});
+    }


Copilot

Pull request overview

Copilot reviewed 43 out of 43 changed files in this pull request and generated 3 comments.

+        # Widen narrow arguments once into lifetime-extended locals so make_[w]format_args (which
+        # requires lvalues) can bind to them. LocalizationArg is identity on non-narrow types.


 #pragma once

 #include "wslutil.h"



@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>


…++ 21) libc++ 21 (Linux SDK 1.23.0) and current MSVC STL delete the std::formatter<char*/const char*/std::string, wchar_t> specializations per C++23 [format.formatter.spec], which collides with WSL's own char->wide formatter specializations in stringshared.h. Remove those specializations and widen narrow arguments at the format boundary instead: - stringshared.h: drop the char->wide formatter specializations; add a WideFormatArg() boundary helper that widens narrow args (char*, const char*, char[N], std::string/std::string_view) to wide, identity otherwise. On non-Windows it is always identity. - generateLocalizationHeader.ps1: wrap every Message*() argument with WideFormatArg via lifetime-extended locals, so localization call sites need no per-site changes. - ExecutionContext.h: normalize narrow user-error messages to wide via ToUserErrorMessage() in the THROW_HR_WITH_USER_ERROR macros. - Fix the remaining direct std::format(L"...") / reporter sinks that pass narrow args (ConsommeNetworking, VersionCommand, ImageProgressCallback, ImageTasks, WslClient version macros, tests). Prefer compile-time STRING_TO_WIDE_STRING for known-narrow literals. - packages.config: bump Microsoft.WSL.LinuxSdk to 1.23.0.

Copilot

Pull request overview

Copilot reviewed 22 out of 22 changed files in this pull request and generated 3 comments.

+inline std::wstring ToUserErrorMessage(std::string_view message)
+{
+    return wsl::shared::string::MultiByteToWide(std::string{message});
+}


+        # Widen narrow arguments once into lifetime-extended locals so make_[w]format_args (which
+        # requires lvalues) can bind to them. LocalizationArg is identity on non-narrow types.


@@ -1,4 +1,4 @@
-<?xml version="1.0" encoding="utf-8"?>
+<?xml version="1.0" encoding="utf-8"?>


benhillis · 2026-07-14T16:43:36Z

Closing in favor of #41071. The fully-conformant approach (removing the char->wide formatter specializations and widening at every boundary) touches ~22 files, because MSVC and libc++ 21 both delete those specializations, so every direct wide std::format narrow call site must change. The minimal Windows-guarded formatter in #41071 is the better tradeoff.

Copilot AI review requested due to automatic review settings July 13, 2026 23:05

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Copilot AI review requested due to automatic review settings July 14, 2026 03:05

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Copilot AI review requested due to automatic review settings July 14, 2026 03:27

benhillis force-pushed the libcxx21-formatter-conformant branch from cf8fb35 to 47a953e Compare July 14, 2026 03:27

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benhillis closed this Jul 14, 2026

benhillis deleted the libcxx21-formatter-conformant branch July 14, 2026 16:43

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Conformant libc++21 formatter refactor (boundary widening) [WIP]#41072

Conformant libc++21 formatter refactor (boundary widening) [WIP]#41072
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		EMIT_USER_WARNING(Localization::MessageConfigUnknownKey(wsl::shared::string::MultiByteToWide(keyName), filePath, fileLine));
		return;

		# Widen narrow arguments once into lifetime-extended locals so make_[w]format_args (which
		# requires lvalues) can bind to them. LocalizationArg is identity on non-narrow types.

		@@ -1,4 +1,4 @@
		<?xml version="1.0" encoding="utf-8"?>
		<?xml version="1.0" encoding="utf-8"?>

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benhillis commented Jul 13, 2026

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