On program startup in C locale, switch to UTF-8 charset if possible

The UTF-8 charset is a superset of the C locale's 7-bit ASCII charset.

Since commit d284ce2fd6 (Initialize character set encoding on program
startup, 2026-02-02, v4.3.0-rc1~40^2) we set a single locale globally.
We can handle archives with UTF-8 paths by running in a locale with a
UTF-8 charset.

Fixes: #25226
Issue: #26903
Issue: #27562

.gitlab/ci/configure_fedora43_makefiles.cmake

@@ -109,6 +109,7 @@ if (NOT "$ENV{CMAKE_CI_NIGHTLY}" STREQUAL "")
   set(CMake_TEST_ISPC "ON" CACHE STRING "")
 endif()
 set(CMake_TEST_LOCALE_CHARSET "ISO-8859-1" CACHE STRING "")
+set(CMake_TEST_LOCALE_C_UTF8 ON CACHE BOOL "")
 set(CMake_TEST_MATH_OVERFLOW ON CACHE BOOL "")
 set(CMake_TEST_Qt5 "ON" CACHE BOOL "")
 set(CMake_TEST_TLS_VERIFY_URL "https://gitlab.kitware.com" CACHE STRING "")

.gitlab/ci/configure_fedora43_ninja.cmake

@@ -5,6 +5,7 @@ if (NOT "$ENV{CMAKE_CI_NIGHTLY}" STREQUAL "")
   set(CMake_TEST_ISPC "ON" CACHE STRING "")
 endif()
 set(CMake_TEST_LOCALE_CHARSET "UTF-8" CACHE STRING "")
+set(CMake_TEST_LOCALE_C_UTF8 ON CACHE BOOL "")
 set(CMake_TEST_MATH_OVERFLOW ON CACHE BOOL "")
 set(CMake_TEST_MODULE_COMPILATION "named,compile_commands,collation,partitions,internal_partitions,export_bmi,install_bmi,shared,bmionly,build_database,import_std23" CACHE STRING "")
 set(CMake_TEST_Rust "ON" CACHE STRING "")

.gitlab/ci/configure_macos_arm64_ninja.cmake

@@ -7,6 +7,7 @@ set(CMake_TEST_FindOpenMP_C "ON" CACHE BOOL "")
 set(CMake_TEST_FindOpenMP_CXX "ON" CACHE BOOL "")
 set(CMake_TEST_GUI "ON" CACHE BOOL "")
 set(CMake_TEST_LOCALE_CHARSET "UTF-8" CACHE STRING "")
+set(CMake_TEST_LOCALE_C_UTF8 ON CACHE BOOL "")
 set(CMake_TEST_MATH_OVERFLOW ON CACHE BOOL "")
 set(CMake_TEST_TLS_VERIFY_URL "https://gitlab.kitware.com" CACHE STRING "")
 set(CMake_TEST_TLS_VERIFY_URL_BAD "https://badtls-expired.kitware.com" CACHE STRING "")

.gitlab/ci/configure_macos_x86_64_ninja.cmake

@@ -10,6 +10,7 @@ if (NOT "$ENV{CMAKE_CI_NIGHTLY}" STREQUAL "")
   set(CMake_TEST_ISPC "ON" CACHE STRING "")
 endif()
 set(CMake_TEST_LOCALE_CHARSET "UTF-8" CACHE STRING "")
+set(CMake_TEST_LOCALE_C_UTF8 ON CACHE BOOL "")
 set(CMake_TEST_MATH_OVERFLOW ON CACHE BOOL "")
 set(CMake_TEST_TLS_VERIFY_URL "https://gitlab.kitware.com" CACHE STRING "")
 set(CMake_TEST_TLS_VERIFY_URL_BAD "https://badtls-expired.kitware.com" CACHE STRING "")

.gitlab/ci/configure_windows_common.cmake

@@ -3,6 +3,7 @@ set(BUILD_CursesDialog ON CACHE BOOL "")
 set(CMAKE_PREFIX_PATH "$ENV{CI_PROJECT_DIR}/.gitlab/qt" CACHE STRING "")
 set(CMake_TEST_Java OFF CACHE BOOL "")
 set(CMake_TEST_LOCALE_CHARSET "UTF-8" CACHE STRING "")
+set(CMake_TEST_LOCALE_C_UTF8 ON CACHE BOOL "")
 set(Python_FIND_REGISTRY NEVER CACHE STRING "")
 
 include("${CMAKE_CURRENT_LIST_DIR}/configure_common.cmake")

Source/cmStdIoInit.cxx

@@ -17,6 +17,9 @@
 
 #  include "cm_fileno.hxx"
 #else
+#  include <cm/string_view>
+#  include <cmext/string_view>
+
 #  include <unistd.h>
 #endif
 
@@ -118,6 +121,17 @@ Globals::Globals()
 #else
   // On non-Windows platforms, we select the user's locale.
   std::setlocale(LC_CTYPE, "");
+
+  // In the "C" locale try switching to a UTF-8 character set.
+  if (char const* locale = std::setlocale(LC_CTYPE, nullptr)) {
+    if (locale == "C"_s || locale == "POSIX"_s) {
+      std::setlocale(LC_CTYPE, "C.UTF-8")
+#  ifdef __APPLE__
+        || std::setlocale(LC_CTYPE, "UTF-8")
+#  endif
+        || std::setlocale(LC_CTYPE, "en_US.UTF-8");
+    }
+  }
 #endif
 }
 

Tests/RunCMake/CMakeLists.txt

@@ -694,6 +694,9 @@ add_RunCMake_test(export)
 if(DEFINED CMake_TEST_LOCALE_CHARSET)
   list(APPEND cmake_host_system_information_ARGS -DCMake_TEST_LOCALE_CHARSET=${CMake_TEST_LOCALE_CHARSET})
 endif()
+if(DEFINED CMake_TEST_LOCALE_C_UTF8)
+  list(APPEND cmake_host_system_information_ARGS -DCMake_TEST_LOCALE_C_UTF8=${CMake_TEST_LOCALE_C_UTF8})
+endif()
 if(CMake_TEST_MSYSTEM_PREFIX)
   list(APPEND cmake_host_system_information_ARGS -DCMake_TEST_MSYSTEM_PREFIX=${CMake_TEST_MSYSTEM_PREFIX})
 endif()

Tests/RunCMake/cmake_host_system_information/RunCMakeTest.cmake

@@ -15,6 +15,14 @@ endif()
 run_cmake_script(Locale)
 unset(RunCMake-stdout-file)
 
+if(CMake_TEST_LOCALE_C_UTF8 OR CMAKE_HOST_WIN32)
+  set(RunCMake-stdout-file Locale-stdout-utf-8.txt)
+  run_cmake_command(LocaleC-UTF8     ${CMAKE_COMMAND} -E env LC_CTYPE=C ${CMAKE_COMMAND}     -P ${RunCMake_SOURCE_DIR}/Locale.cmake)
+  run_cmake_command(LocaleBad-UTF8   ${CMAKE_COMMAND} -E env LC_CTYPE=Bad ${CMAKE_COMMAND}   -P ${RunCMake_SOURCE_DIR}/Locale.cmake)
+  run_cmake_command(LocalePOSIX-UTF8 ${CMAKE_COMMAND} -E env LC_CTYPE=POSIX ${CMAKE_COMMAND} -P ${RunCMake_SOURCE_DIR}/Locale.cmake)
+  unset(RunCMake-stdout-file)
+endif()
+
 run_cmake(UnitTest)
 run_cmake(Exherbo)
 run_cmake(Ubuntu)