From 87308cc72fc725f535a78d91b4e02e98a8289bd7 Mon Sep 17 00:00:00 2001
From: ClausKlein <claus.klein@arcormail.de>
Date: Sat, 20 Jun 2026 19:04:15 +0200
Subject: [PATCH 01/14] Bump version: 1.91.0-rc0 -> 1.91.0-rc1

---
 .bumpversion.cfg         |  2 +-
 .bumpversion.md          |  2 +-
 .pre-commit-config.yaml  |  2 +-
 CMakeLists.txt           | 29 ++++++++++++++++-------------
 CMakeReleasePresets.json |  4 ++--
 README.md                |  2 +-
 examples/CMakeLists.txt  | 10 +++++-----
 requirements.txt         |  8 ++++----
 8 files changed, 31 insertions(+), 28 deletions(-)

diff --git a/.bumpversion.cfg b/.bumpversion.cfg
index cce5abd9..262d6fe2 100644
--- a/.bumpversion.cfg
+++ b/.bumpversion.cfg
@@ -4,7 +4,7 @@ message = Bump version: {current_version} -> {new_version}
 tag_message = Release v{new_version}
 tag_name = v{new_version}
 tag = True
-current_version = 1.90.0-rc2
+current_version = 1.91.0-rc1
 parse = (?P<major>\d+)\.(?P<minor>\d+)\.(?P<patch>\d+)([-](?P<release>(dev|rc))+(?P<build>\d+))?
 serialize = 
 	{major}.{minor}.{patch}-{release}{build}
diff --git a/.bumpversion.md b/.bumpversion.md
index 484db6d8..fc8182de 100644
--- a/.bumpversion.md
+++ b/.bumpversion.md
@@ -50,7 +50,7 @@ Start with an initial release, say `1.0.0`.
    `ValueError: The part has already the maximum value among ['dev', 'rc', 'ga'] and cannot be bumped`.
 * A whole version string may be set with:
 
-    bumpversion --new-version 1.90.0 patch --no-commit --no-tag --allow-dirty
+    bumpversion --new-version 1.91.0-rc0 release --no-commit --no-tag --allow-dirty
 
 # Reference
 
diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 6a597b57..10b94ece 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -15,7 +15,7 @@ repos:
   # This brings in a portable version of clang-format.
   # See also: https://github.com/ssciwr/clang-format-wheel
   - repo: https://github.com/pre-commit/mirrors-clang-format
-    rev: v22.1.0
+    rev: v22.1.5
     hooks:
       - id: clang-format
         types_or: [c++, c, json]
diff --git a/CMakeLists.txt b/CMakeLists.txt
index 20eda552..d7739d7b 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,7 +1,7 @@
-cmake_minimum_required(VERSION 3.30...4.3)
+cmake_minimum_required(VERSION 3.30...4.4)
 
 # gersemi: off
-project(Boost-CMake LANGUAGES CXX VERSION 1.90.0.2)
+project(Boost-CMake LANGUAGES CXX VERSION 1.91.0.1)
 # gersemi: on
 
 if(PROJECT_IS_TOP_LEVEL)
@@ -75,16 +75,18 @@ set(BOOST_LIBS_REQUIRED headers)
 set(BOOST_EXCLUDE_LIBRARIES
     cobalt
     fiber
+    graph
     iostreams
     locale
     log
     json
+    serialization
     timer
     wave
 )
 
 if(PROJECT_IS_TOP_LEVEL AND NOT BOOST_BUILD_ALL)
-    # NOTE: Compiled libs without warnings OSX with clang v21.1.7! CK
+    # NOTE: Compiled libs without warnings OSX with clang v22.1.7! CK
     list(
         APPEND BOOST_INCLUDE_LIBRARIES
         ${BOOST_LIBS_REQUIRED}
@@ -99,7 +101,7 @@ if(PROJECT_IS_TOP_LEVEL AND NOT BOOST_BUILD_ALL)
         exception
         filesystem # FIXME: Warnings with g++-15 on OSX! CK
         geometry # deps on any
-        graph # deps on any, serialization
+        # TODO(CK): graph # deps on any, serialization
         json
         math
         optional
@@ -110,7 +112,7 @@ if(PROJECT_IS_TOP_LEVEL AND NOT BOOST_BUILD_ALL)
         property_map # deps on any
         property_tree # deps on any
         regex
-        serialization
+        # TODO(CK): serialization
         signals2
         system
         test
@@ -131,7 +133,7 @@ endif()
 # ---- Add/prepare dependency handling via CPM ----
 # see https://github.com/TheLartians/CPM.cmake for more info
 
-set(BOOST_SUBDIR boost_1_90_0)
+set(BOOST_SUBDIR boost-1.91.0-1)
 if(EXISTS "$ENV{CPM_SOURCE_CACHE}/${BOOST_SUBDIR}")
     set(FETCHCONTENT_SOURCE_DIR_BOOST
         "$ENV{CPM_SOURCE_CACHE}/${BOOST_SUBDIR}"
@@ -147,14 +149,15 @@ if(FETCHCONTENT_SOURCE_DIR_BOOST)
     )
 endif()
 
-set(BOOST_VERSION 1.90.0)
+set(BOOST_VERSION 1.91.0)
+set(BOOST_FILE_VERSION ${BOOST_VERSION}-1)
 set(BOOST_URL
-    "https://github.com/boostorg/boost/releases/download/boost-${BOOST_VERSION}/boost-${BOOST_VERSION}-cmake.7z"
+    "https://github.com/boostorg/boost/releases/download/boost-${BOOST_FILE_VERSION}/boost-${BOOST_FILE_VERSION}-cmake.7z"
     CACHE STRING
     "Boost download URL"
 )
 set(BOOST_URL_SHA256
-    "218e74c4aa362a994b7b7a23b2920f455a00205c656405fcf262cf60b8871921"
+    "29c7d4f4ac36ad853b6765d03571ea60d90286775df026b4efd9f3281131972b"
     CACHE STRING
     "Boost download URL SHA256 checksum"
 )
@@ -166,9 +169,9 @@ FetchContent_Declare(
     URL ${BOOST_URL}
     URL_HASH SHA256=${BOOST_URL_SHA256}
     DOWNLOAD_EXTRACT_TIMESTAMP ON
-    PATCH_COMMAND
-        git apply -p0
-        ${CMAKE_CURRENT_SOURCE_DIR}/patches/${BOOST_VERSION}/cmake-01.patch
+    # PATCH_COMMAND
+    # git apply -p0
+    # ${CMAKE_CURRENT_SOURCE_DIR}/patches/${BOOST_VERSION}/cmake-01.patch
     SYSTEM
     ${EXCLUDE_FROM_ALL}
 )
@@ -178,7 +181,7 @@ if(NOT Boost_POPULATED)
     message(STATUS "Fetching Boost")
     FetchContent_MakeAvailable(Boost)
     message(STATUS "Fetching Boost - done")
-    set(BOOST_SOURCE ${boost_SOURCE_DIR})
+    set(BOOST_SOURCE ${Boost_SOURCE_DIR})
     message(STATUS "Boost_SOURCE_DIR=${Boost_SOURCE_DIR}")
 endif()
 
diff --git a/CMakeReleasePresets.json b/CMakeReleasePresets.json
index e78fdbff..fbcb6dd9 100644
--- a/CMakeReleasePresets.json
+++ b/CMakeReleasePresets.json
@@ -22,8 +22,8 @@
         "BOOST_STANDALONE": true,
         "BUILD_SHARED_LIBS": true,
         "CMAKE_BUILD_TYPE": "Release",
-        "CMAKE_CXX_EXTENSIONS": true,
-        "CMAKE_CXX_STANDARD": "20",
+        "CMAKE_CXX_EXTENSIONS": false,
+        "CMAKE_CXX_STANDARD": "23",
         "CMAKE_CXX_STANDARD_REQUIRED": true,
         "CMAKE_DEBUG_POSTFIX": "-d",
         "CMAKE_EXPORT_COMPILE_COMMANDS": true,
diff --git a/README.md b/README.md
index 1cd424ae..1563df4b 100644
--- a/README.md
+++ b/README.md
@@ -23,7 +23,7 @@ option(BUILD_SHARED_LIBS "Build shared libraries" NO)
 set(BOOST_INCLUDE_LIBRARIES any pfr beast filesystem headers)
 set(BOOST_USE_MODULES ON)
 set(Boost_VERBOSE ON)
-CPMAddPackage("gh:ClausKlein/boost-cmake#v1.90.0-rc2")
+CPMAddPackage("gh:ClausKlein/boost-cmake#v1.91.0-rc1")
 
 target_include_directories(
   ${PROJECT_NAME} PUBLIC $<BUILD_INTERFACE:${PROJECT_SOURCE_DIR}/include>
diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index e1b71f9f..ab1290e7 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -1,6 +1,6 @@
-cmake_minimum_required(VERSION 3.30...4.3)
+cmake_minimum_required(VERSION 3.30...4.4)
 
-# test boost-cmake v1.90.0-rc2
+# test boost-cmake v1.91.0-rc1
 project(BoostExamples LANGUAGES CXX)
 
 enable_testing()
@@ -12,7 +12,7 @@ if(PROJECT_IS_TOP_LEVEL)
 
     set(CMAKE_SKIP_TEST_ALL_DEPENDENCY NO)
     if(NOT DEFINED CMAKE_CXX_STANDARD)
-        set(CMAKE_CXX_STANDARD 20)
+        set(CMAKE_CXX_STANDARD 23)
     endif()
     if(NOT DEFINED CMAKE_PREFIX_PATH)
         list(APPEND CMAKE_PREFIX_PATH ../stagedir)
@@ -36,7 +36,7 @@ if(PROJECT_IS_TOP_LEVEL)
 
         set(BUILD_SHARED_LIBS ON)
         set(BOOST_SKIP_INSTALL_RULES ${CMAKE_SKIP_INSTALL_RULES})
-        set(BOOST_VERSION 1.90.0)
+        set(BOOST_VERSION 1.91.0-1)
 
         include(FetchContent)
         FetchContent_Declare(
@@ -44,7 +44,7 @@ if(PROJECT_IS_TOP_LEVEL)
             URL
                 https://github.com/boostorg/boost/releases/download/boost-${BOOST_VERSION}/boost-${BOOST_VERSION}-cmake.7z
             URL_HASH
-                SHA256=218e74c4aa362a994b7b7a23b2920f455a00205c656405fcf262cf60b8871921
+                SHA256:29c7d4f4ac36ad853b6765d03571ea60d90286775df026b4efd9f3281131972b
             EXCLUDE_FROM_ALL
             SYSTEM
             FIND_PACKAGE_ARGS
diff --git a/requirements.txt b/requirements.txt
index 4f26a506..09fad138 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -17,10 +17,10 @@ pre-commit
 ### cmake build context
 bump2version>=1.0.1
 check-jsonschema>=0.31.2
-cmake-format>=0.6.13
+# cmake-format>=0.6.13
 cmake>=3.30
-codespell>=2.4.1
+codespell>=2.4.2
 conan>=2.13.0
-gcovr>=8.3
-ninja>=1.11
+gcovr>=8.6
+ninja>=1.13
 yamllint>=1.35

From 9496197ef307c98c5b66011c13c6e62511ad11b1 Mon Sep 17 00:00:00 2001
From: ClausKlein <claus.klein@arcormail.de>
Date: Sat, 20 Jun 2026 19:35:42 +0200
Subject: [PATCH 02/14] Prevent unity_builds on windows

---
 .github/workflows/cmake.yml | 2 +-
 CMakeLists.txt              | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/.github/workflows/cmake.yml b/.github/workflows/cmake.yml
index 6d4b49d0..d6616ad9 100644
--- a/.github/workflows/cmake.yml
+++ b/.github/workflows/cmake.yml
@@ -36,7 +36,7 @@ jobs:
       - name: Setup build environment
         uses: lukka/get-cmake@latest
         with:
-          cmakeVersion: "~4.2.1"
+          cmakeVersion: "~4.3.4"
           ninjaVersion: "^1.13.0"
 
       - name: Setup MSVC
diff --git a/CMakeLists.txt b/CMakeLists.txt
index d7739d7b..ebf133c9 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -34,7 +34,7 @@ if(BOOST_USE_MODULES AND NOT UNIX)
     set(BUILD_SHARED_LIBS OFF)
 endif()
 
-if(NOT BOOST_USE_MODULES)
+if(NOT BOOST_USE_MODULES AND UNIX)
     message(STATUS "Use CMAKE_UNITY_BUILD")
     set(CMAKE_UNITY_BUILD ON)
 endif()

From 902dde11ab1af3bcbbcdc5800c11ac16a7556c4c Mon Sep 17 00:00:00 2001
From: ClausKlein <claus.klein@arcormail.de>
Date: Sun, 21 Jun 2026 14:48:19 +0200
Subject: [PATCH 03/14] Test BOOST_USE_MODULES too

---
 CMakeLists.txt                | 12 ++---
 CMakeReleasePresets.json      |  2 +-
 GNUmakefile                   |  8 ++--
 examples/CMakeLists.txt       |  8 ++--
 patches/1.91.0/cmake-01.patch | 89 +++++++++++++++++++++++++++++++++++
 5 files changed, 104 insertions(+), 15 deletions(-)
 create mode 100644 patches/1.91.0/cmake-01.patch

diff --git a/CMakeLists.txt b/CMakeLists.txt
index ebf133c9..6b7ba28d 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -75,12 +75,10 @@ set(BOOST_LIBS_REQUIRED headers)
 set(BOOST_EXCLUDE_LIBRARIES
     cobalt
     fiber
-    graph
     iostreams
     locale
     log
     json
-    serialization
     timer
     wave
 )
@@ -101,7 +99,7 @@ if(PROJECT_IS_TOP_LEVEL AND NOT BOOST_BUILD_ALL)
         exception
         filesystem # FIXME: Warnings with g++-15 on OSX! CK
         geometry # deps on any
-        # TODO(CK): graph # deps on any, serialization
+        graph # deps on any, serialization
         json
         math
         optional
@@ -112,7 +110,7 @@ if(PROJECT_IS_TOP_LEVEL AND NOT BOOST_BUILD_ALL)
         property_map # deps on any
         property_tree # deps on any
         regex
-        # TODO(CK): serialization
+        serialization
         signals2
         system
         test
@@ -169,9 +167,9 @@ FetchContent_Declare(
     URL ${BOOST_URL}
     URL_HASH SHA256=${BOOST_URL_SHA256}
     DOWNLOAD_EXTRACT_TIMESTAMP ON
-    # PATCH_COMMAND
-    # git apply -p0
-    # ${CMAKE_CURRENT_SOURCE_DIR}/patches/${BOOST_VERSION}/cmake-01.patch
+    PATCH_COMMAND
+        git apply -p0
+        ${CMAKE_CURRENT_SOURCE_DIR}/patches/${BOOST_VERSION}/cmake-01.patch
     SYSTEM
     ${EXCLUDE_FROM_ALL}
 )
diff --git a/CMakeReleasePresets.json b/CMakeReleasePresets.json
index fbcb6dd9..893c7eed 100644
--- a/CMakeReleasePresets.json
+++ b/CMakeReleasePresets.json
@@ -23,7 +23,7 @@
         "BUILD_SHARED_LIBS": true,
         "CMAKE_BUILD_TYPE": "Release",
         "CMAKE_CXX_EXTENSIONS": false,
-        "CMAKE_CXX_STANDARD": "23",
+        "CMAKE_CXX_STANDARD": "20",
         "CMAKE_CXX_STANDARD_REQUIRED": true,
         "CMAKE_DEBUG_POSTFIX": "-d",
         "CMAKE_EXPORT_COMPILE_COMMANDS": true,
diff --git a/GNUmakefile b/GNUmakefile
index 2b96a3c4..61835b0c 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -21,8 +21,8 @@ ifeq (${hostSystemName},Darwin)
   export GCC_PREFIX:=$(shell brew --prefix gcc)
   export GCC_DIR:=$(shell realpath ${GCC_PREFIX})
 
-# export CMAKE_CXX_STDLIB_MODULES_JSON:=${GCC_DIR}/lib/gcc/current/libstdc++.modules.json
-# export CXX::=g++-15
+  # export CMAKE_CXX_STDLIB_MODULES_JSON:=${GCC_DIR}/lib/gcc/current/libstdc++.modules.json
+  # export CXX::=g++-15
   # export CXXFLAGS:=-stdlib=libstdc++
   # export GCOV:="gcov"
 else ifeq (${hostSystemName},Linux)
@@ -42,13 +42,13 @@ examples: # XXX install
 	ninja -C build test -v
 
 fresh:
-	cmake --workflow --preset Release --log-level=VERBOSE --fresh
+	cmake --workflow --preset Release --fresh
 
 compile_commands.json: build/Release/compile_commands.json
 	ln -sf $< .
 
 build/Release/compile_commands.json: GNUmakefile CMakeLists.txt
-	cmake --preset Release --log-level=VERBOSE
+	cmake --preset Release --log-level=VERBOSE -D BOOST_USE_MODULES=ON
 
 build: compile_commands.json
 	cmake --build --preset Release
diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index ab1290e7..a4d4b1d3 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -10,9 +10,11 @@ enable_testing()
 if(PROJECT_IS_TOP_LEVEL)
     include(../cmake/ccache.cmake)
 
-    set(CMAKE_SKIP_TEST_ALL_DEPENDENCY NO)
+    set(CMAKE_SKIP_TEST_ALL_DEPENDENCY OFF)
+    set(CMAKE_CXX_EXTENSIONS OFF)
+    set(CMAKE_CXX_STANDARD_REQUIRED ON)
     if(NOT DEFINED CMAKE_CXX_STANDARD)
-        set(CMAKE_CXX_STANDARD 23)
+        set(CMAKE_CXX_STANDARD 20)
     endif()
     if(NOT DEFINED CMAKE_PREFIX_PATH)
         list(APPEND CMAKE_PREFIX_PATH ../stagedir)
@@ -34,7 +36,7 @@ if(PROJECT_IS_TOP_LEVEL)
 
         set(CMAKE_SKIP_INSTALL_RULES ON)
 
-        set(BUILD_SHARED_LIBS ON)
+        set(BUILD_SHARED_LIBS OFF)
         set(BOOST_SKIP_INSTALL_RULES ${CMAKE_SKIP_INSTALL_RULES})
         set(BOOST_VERSION 1.91.0-1)
 
diff --git a/patches/1.91.0/cmake-01.patch b/patches/1.91.0/cmake-01.patch
new file mode 100644
index 00000000..955da17d
--- /dev/null
+++ b/patches/1.91.0/cmake-01.patch
@@ -0,0 +1,89 @@
+--- tools/cmake/include/BoostInstall.cmake.old	2026-01-08 11:26:12.013228000 +0100
++++ tools/cmake/include/BoostInstall.cmake	2026-01-08 13:59:42.824649975 +0100
+@@ -337,6 +337,22 @@
+     string(APPEND CONFIG_INSTALL_DIR "-static")
+   endif()
+ 
++  if(NOT CMAKE_VERSION VERSION_LESS 3.28)
++    get_target_property(INTERFACE_CXX_MODULE_SETS ${LIB} INTERFACE_CXX_MODULE_SETS)
++    if(INTERFACE_CXX_MODULE_SETS)
++      boost_message(DEBUG "boost_install_target: '${__TARGET}' has INTERFACE_CXX_MODULE_SETS=${INTERFACE_CXX_MODULE_SETS}")
++      set(__INSTALL_CXX_MODULES FILE_SET ${INTERFACE_CXX_MODULE_SETS} DESTINATION ${CMAKE_INSTALL_DATADIR})
++    endif()
++  endif()
++
++  if(NOT CMAKE_VERSION VERSION_LESS 3.23)
++    get_target_property(INTERFACE_HEADER_SETS ${LIB} INTERFACE_HEADER_SETS)
++    if(INTERFACE_HEADER_SETS)
++      boost_message(DEBUG "boost_install_target: '${__TARGET}' has INTERFACE_HEADER_SETS=${INTERFACE_HEADER_SETS}")
++      set(__INSTALL_HEADER_SETS FILE_SET ${INTERFACE_HEADER_SETS})
++    endif()
++  endif()
++
+   install(TARGETS ${LIB} EXPORT ${LIB}-targets
+     # explicit destination specification required for 3.13, 3.14 no longer needs it
+     RUNTIME DESTINATION "${CMAKE_INSTALL_BINDIR}"
+@@ -344,8 +360,18 @@
+     ARCHIVE DESTINATION "${CMAKE_INSTALL_LIBDIR}"
+     PRIVATE_HEADER DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
+     PUBLIC_HEADER DESTINATION "${CMAKE_INSTALL_INCLUDEDIR}"
++    # explicit needed if used starting with cmake v3.28
++    # XXX FILE_SET CXX_MODULES DESTINATION "${CMAKE_INSTALL_DATADIR}"
++    ${__INSTALL_CXX_MODULES}
++    # Any module files from C++ modules from PUBLIC sources in a file set of type CXX_MODULES will be installed to the given DESTINATION.
++    # FIXME: why does this silently not work here? CK
++    # XXX CXX_MODULES_BMI DESTINATION ${CONFIG_INSTALL_DIR}/bmi-${CMAKE_CXX_COMPILER_ID}_$<CONFIG>
++    # explicit needed if used starting with cmake v3.23
++    # XXX FILE_SET HEADERS
++    ${__INSTALL_HEADER_SETS}
+   )
+ 
++  # TODO(CK): what is this for?
+   export(TARGETS ${LIB} NAMESPACE Boost:: FILE export/${LIB}-targets.cmake)
+ 
+   if(MSVC)
+@@ -362,7 +388,9 @@
+     __boost_install_update_sources(${LIB} ${__EXTRA_DIRECTORY} ${__EXTRA_INSTALL_DIRECTORY})
+   endif()
+ 
+-  install(EXPORT ${LIB}-targets DESTINATION "${CONFIG_INSTALL_DIR}" NAMESPACE Boost:: FILE ${LIB}-targets.cmake)
++  install(EXPORT ${LIB}-targets DESTINATION "${CONFIG_INSTALL_DIR}" NAMESPACE Boost:: FILE ${LIB}-targets.cmake
++    # XXX CXX_MODULES_DIRECTORY .
++  )
+ 
+   set_target_properties(${LIB} PROPERTIES _boost_is_installed ON)
+ 
+--- tools/cmake/include/BoostRoot.cmake.old	2026-01-08 11:31:49.525898365 +0100
++++ tools/cmake/include/BoostRoot.cmake	2026-01-08 11:31:04.000000000 +0100
+@@ -123,7 +123,9 @@
+     set(BOOST_SKIP_INSTALL_RULES ON)
+   endif()
+ 
+-  set(BUILD_TESTING OFF)
++  if(NOT BOOST_BUILD_TESTING)
++    set(BUILD_TESTING OFF)
++  endif()
+ 
+ endif()
+ 
+@@ -303,6 +305,9 @@
+ 
+ endmacro()
+ 
++# C++20 modules
++option(BOOST_USE_MODULES "Build Boost as a collection of C++20 modules (unstable)" OFF)
++
+ #
+ 
+ file(GLOB __boost_libraries RELATIVE "${BOOST_SUPERPROJECT_SOURCE_DIR}/libs" "${BOOST_SUPERPROJECT_SOURCE_DIR}/libs/*/CMakeLists.txt" "${BOOST_SUPERPROJECT_SOURCE_DIR}/libs/numeric/*/CMakeLists.txt")
+--- libs/serialization/CMakeLists.txt.old	2025-12-10 17:42:32.995666000 +0100
++++ libs/serialization/CMakeLists.txt	2026-01-08 18:20:48.675263056 +0100
+@@ -6,6 +6,8 @@
+ 
+ project(boost_serialization VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)
+ 
++set(CMAKE_UNITY_BUILD OFF)
++
+ add_library(boost_serialization
+   src/archive_exception.cpp
+   src/basic_archive.cpp

From 01a4842f709056761754f09a6c107f4adf7e1016 Mon Sep 17 00:00:00 2001
From: ClausKlein <claus.klein@arcormail.de>
Date: Sun, 21 Jun 2026 15:01:32 +0200
Subject: [PATCH 04/14] Update Readme and CPM.cmake module

---
 README.md       | 6 +++---
 cmake/CPM.cmake | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

diff --git a/README.md b/README.md
index 1563df4b..e839e1ca 100644
--- a/README.md
+++ b/README.md
@@ -41,8 +41,8 @@ Boost will automatically be downloaded from https://github.com/boostorg/boost/re
 If that is not acceptable to you, you can use an alternate Boost version, apply
 custom patches or just mirror the current archive in your internal network like so:
 ```
-set(BOOST_URL http://internal.mirror/boost-1.90.0-cmake.7z)
-set(BOOST_URL_SHA256 218e74c4aa362a994b7b7a23b2920f455a00205c656405fcf262cf60b8871921)
+set(BOOST_URL http://internal.mirror/boost-1.91.0-1-cmake.7z)
+set(BOOST_URL_SHA256 29c7d4f4ac36ad853b6765d03571ea60d90286775df026b4efd9f3281131972b)
 ```
 
 If you have boost-cmake and Boost cmake sources files already available and want to point to them, you can use the following:
@@ -70,7 +70,7 @@ packageProject(
   INCLUDE_HEADER_PATTERN "*.h"
   DISABLE_VERSION_SUFFIX YES
   COMPATIBILITY SameMajorVersion
-  DEPENDENCIES "Boost 1.90"
+  DEPENDENCIES "Boost 1.91"
 )
 
 include(CPack)
diff --git a/cmake/CPM.cmake b/cmake/CPM.cmake
index 70ce5879..cf1772ed 100644
--- a/cmake/CPM.cmake
+++ b/cmake/CPM.cmake
@@ -2,9 +2,9 @@
 #
 # SPDX-FileCopyrightText: Copyright (c) 2019-2023 Lars Melchior and contributors
 
-set(CPM_DOWNLOAD_VERSION 0.42.0)
+set(CPM_DOWNLOAD_VERSION 0.42.3)
 set(CPM_HASH_SUM
-    "2020b4fc42dba44817983e06342e682ecfc3d2f484a581f11cc5731fbe4dce8a"
+    "a609e875fd532b067174250f6abbc3dac22fe2d64869783fb1e80bda1625c844"
 )
 
 if(CPM_SOURCE_CACHE)

From 2e71c0a145419c592c627171037a311b5424ec21 Mon Sep 17 00:00:00 2001
From: ClausKlein <claus.klein@arcormail.de>
Date: Sun, 21 Jun 2026 16:22:48 +0200
Subject: [PATCH 05/14] Test BOOST_USE_MODULES on CI

---
 .github/workflows/cmake.yml | 2 +-
 GNUmakefile                 | 2 +-
 2 files changed, 2 insertions(+), 2 deletions(-)

diff --git a/.github/workflows/cmake.yml b/.github/workflows/cmake.yml
index d6616ad9..22331fd4 100644
--- a/.github/workflows/cmake.yml
+++ b/.github/workflows/cmake.yml
@@ -52,7 +52,7 @@ jobs:
           compiler: llvm
 
       - name: Configure CMake
-        run: cmake --preset ${{env.BUILD_TYPE}} --log-level=VERBOSE
+        run: cmake --preset ${{env.BUILD_TYPE}} --log-level=VERBOSE -D BOOST_USE_MODULES=ON
 
       - name: Build
         # Build your program with the given configuration
diff --git a/GNUmakefile b/GNUmakefile
index 61835b0c..d698da84 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -22,7 +22,7 @@ ifeq (${hostSystemName},Darwin)
   export GCC_DIR:=$(shell realpath ${GCC_PREFIX})
 
   # export CMAKE_CXX_STDLIB_MODULES_JSON:=${GCC_DIR}/lib/gcc/current/libstdc++.modules.json
-  # export CXX::=g++-15
+  # export CXX:=g++-15
   # export CXXFLAGS:=-stdlib=libstdc++
   # export GCOV:="gcov"
 else ifeq (${hostSystemName},Linux)

From cc6df655d11cf6f7aaf56e0475ddb8887a97f719 Mon Sep 17 00:00:00 2001
From: ClausKlein <claus.klein@arcormail.de>
Date: Mon, 22 Jun 2026 20:25:23 +0200
Subject: [PATCH 06/14] Patch boost_any too

---
 CMakeLists.txt                       | 26 +++++++++++++++++++++++++-
 CMakeReleasePresets.json             |  2 +-
 GNUmakefile                          |  3 ++-
 examples/CMakeLists.txt              |  2 +-
 examples/any_module_usage_sample.cpp |  2 +-
 patches/1.91.0/cmake-01.patch        | 22 ++++++++++++++++++++++
 6 files changed, 52 insertions(+), 5 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 6b7ba28d..879add12 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,5 +1,29 @@
 cmake_minimum_required(VERSION 3.30...4.4)
 
+if(CMAKE_VERSION VERSION_GREATER_EQUAL "3.30.0")
+    if(CMAKE_VERSION VERSION_LESS "3.31.8")
+        set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
+            "0e5b6991-d74f-4b3d-a41c-cf096e0b2508"
+        )
+    elseif(CMAKE_VERSION VERSION_LESS "4.0.0")
+        set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
+            "d0edc3af-4c50-42ea-a356-e2862fe7a444"
+        )
+    elseif(CMAKE_VERSION VERSION_LESS "4.0.3")
+        set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
+            "a9e1cf81-9932-4810-974b-6eccaf14e457"
+        )
+    elseif(CMAKE_VERSION VERSION_LESS "4.3.0")
+        set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
+            "d0edc3af-4c50-42ea-a356-e2862fe7a444"
+        )
+    elseif(CMAKE_VERSION VERSION_LESS "4.4.0")
+        set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
+            "451f2fe2-a8a2-47c3-bc32-94786d8fc91b"
+        )
+    endif()
+endif()
+
 # gersemi: off
 project(Boost-CMake LANGUAGES CXX VERSION 1.91.0.1)
 # gersemi: on
@@ -12,7 +36,7 @@ if(PROJECT_IS_TOP_LEVEL)
     option(BOOST_USE_MODULES "Do build with CXX_MODULES if possible" OFF)
     if(BOOST_USE_MODULES)
         set(CMAKE_CXX_SCAN_FOR_MODULES ON)
-        set(CMAKE_CXX_MODULE_STD OFF) # TODO(CK): not yet!
+        # XXX set(CMAKE_CXX_MODULE_STD ON) # TODO(CK): not yet!
     else()
         set(CMAKE_CXX_SCAN_FOR_MODULES OFF)
     endif()
diff --git a/CMakeReleasePresets.json b/CMakeReleasePresets.json
index 893c7eed..fbcb6dd9 100644
--- a/CMakeReleasePresets.json
+++ b/CMakeReleasePresets.json
@@ -23,7 +23,7 @@
         "BUILD_SHARED_LIBS": true,
         "CMAKE_BUILD_TYPE": "Release",
         "CMAKE_CXX_EXTENSIONS": false,
-        "CMAKE_CXX_STANDARD": "20",
+        "CMAKE_CXX_STANDARD": "23",
         "CMAKE_CXX_STANDARD_REQUIRED": true,
         "CMAKE_DEBUG_POSTFIX": "-d",
         "CMAKE_EXPORT_COMPILE_COMMANDS": true,
diff --git a/GNUmakefile b/GNUmakefile
index d698da84..96af1465 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -48,7 +48,8 @@ compile_commands.json: build/Release/compile_commands.json
 	ln -sf $< .
 
 build/Release/compile_commands.json: GNUmakefile CMakeLists.txt
-	cmake --preset Release --log-level=VERBOSE -D BOOST_USE_MODULES=ON
+	cmake --preset Release --log-level=VERBOSE -D BOOST_USE_MODULES=ON -D CMAKE_CXX_MODULE_STD=ON \
+	-D CMAKE_CXX_STDLIB_MODULES_JSON=${CMAKE_CXX_STDLIB_MODULES_JSON}
 
 build: compile_commands.json
 	cmake --build --preset Release
diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index a4d4b1d3..51cbe6f8 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -14,7 +14,7 @@ if(PROJECT_IS_TOP_LEVEL)
     set(CMAKE_CXX_EXTENSIONS OFF)
     set(CMAKE_CXX_STANDARD_REQUIRED ON)
     if(NOT DEFINED CMAKE_CXX_STANDARD)
-        set(CMAKE_CXX_STANDARD 20)
+        set(CMAKE_CXX_STANDARD 23)
     endif()
     if(NOT DEFINED CMAKE_PREFIX_PATH)
         list(APPEND CMAKE_PREFIX_PATH ../stagedir)
diff --git a/examples/any_module_usage_sample.cpp b/examples/any_module_usage_sample.cpp
index cc0d3369..b1faf5f0 100644
--- a/examples/any_module_usage_sample.cpp
+++ b/examples/any_module_usage_sample.cpp
@@ -47,7 +47,7 @@ namespace {
 auto main() -> int {
   boost::any const a = 42;
 #ifdef BOOST_ANY_USE_STD_MODULE
-  std::println(stdout, "{}", any_to_string<int, double, std::string>(a));
+  std::println(std::cout, "{}", any_to_string<int, double, std::string>(a));
 #else
   std::cout << any_to_string<int, double, std::string>(a) << '\n';
 #endif
diff --git a/patches/1.91.0/cmake-01.patch b/patches/1.91.0/cmake-01.patch
index 955da17d..93fc4a66 100644
--- a/patches/1.91.0/cmake-01.patch
+++ b/patches/1.91.0/cmake-01.patch
@@ -87,3 +87,25 @@
  add_library(boost_serialization
    src/archive_exception.cpp
    src/basic_archive.cpp
+--- libs/any/CMakeLists.txt.old	2026-06-22 19:54:36.000000000 
++++ libs/any/CMakeLists.txt	2026-06-22 19:59:27.000000000 
+@@ -14,16 +14,17 @@
+     )
+ 
+     target_compile_features(boost_any PUBLIC cxx_std_20)
+     target_compile_definitions(boost_any PUBLIC BOOST_USE_MODULES)
+     set(__boost_cxx_standard ${CMAKE_CXX_STANDARD})
+     if (NOT __boost_cxx_standard)
+-        set(__boost_cxx_standard 20)
++        set(__boost_cxx_standard 23)
+     endif()
+     if (CMAKE_CXX_COMPILER_IMPORT_STD AND ${__boost_cxx_standard} IN_LIST CMAKE_CXX_COMPILER_IMPORT_STD)
+-        target_compile_definitions(boost_any PRIVATE BOOST_ANY_USE_STD_MODULE)
++        target_compile_definitions(boost_any PUBLIC BOOST_ANY_USE_STD_MODULE)
++        target_compile_features(boost_any PUBLIC cxx_std_${__boost_cxx_standard})
+         message(STATUS "Using `import std;`")
+     else()
+         message(STATUS "`import std;` is not available")
+     endif()
+     set(__scope PUBLIC)
+ else()

From 85753e733cb45b04a2b8122f4f4ef1c82f1b2c24 Mon Sep 17 00:00:00 2001
From: ClausKlein <claus.klein@arcormail.de>
Date: Tue, 23 Jun 2026 06:42:10 +0200
Subject: [PATCH 07/14] Disable CMAKE_CXX_MODULE_STD again

---
 CMakeLists.txt           | 3 ++-
 CMakeReleasePresets.json | 2 +-
 GNUmakefile              | 1 +
 examples/CMakeLists.txt  | 2 +-
 4 files changed, 5 insertions(+), 3 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 879add12..0ee54adf 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -36,7 +36,7 @@ if(PROJECT_IS_TOP_LEVEL)
     option(BOOST_USE_MODULES "Do build with CXX_MODULES if possible" OFF)
     if(BOOST_USE_MODULES)
         set(CMAKE_CXX_SCAN_FOR_MODULES ON)
-        # XXX set(CMAKE_CXX_MODULE_STD ON) # TODO(CK): not yet!
+        set(CMAKE_CXX_MODULE_STD OFF) # TODO(CK): not yet!
     else()
         set(CMAKE_CXX_SCAN_FOR_MODULES OFF)
     endif()
@@ -125,6 +125,7 @@ if(PROJECT_IS_TOP_LEVEL AND NOT BOOST_BUILD_ALL)
         geometry # deps on any
         graph # deps on any, serialization
         json
+        lexical_cast # TODO(CK): use BOOST_USE_MODULES but import std wrong!
         math
         optional
         parser # deps on type_index
diff --git a/CMakeReleasePresets.json b/CMakeReleasePresets.json
index fbcb6dd9..893c7eed 100644
--- a/CMakeReleasePresets.json
+++ b/CMakeReleasePresets.json
@@ -23,7 +23,7 @@
         "BUILD_SHARED_LIBS": true,
         "CMAKE_BUILD_TYPE": "Release",
         "CMAKE_CXX_EXTENSIONS": false,
-        "CMAKE_CXX_STANDARD": "23",
+        "CMAKE_CXX_STANDARD": "20",
         "CMAKE_CXX_STANDARD_REQUIRED": true,
         "CMAKE_DEBUG_POSTFIX": "-d",
         "CMAKE_EXPORT_COMPILE_COMMANDS": true,
diff --git a/GNUmakefile b/GNUmakefile
index 96af1465..872024de 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -47,6 +47,7 @@ fresh:
 compile_commands.json: build/Release/compile_commands.json
 	ln -sf $< .
 
+# NOTE: Works only yet on OSX with clang v22.1.7 with this arguments! CK
 build/Release/compile_commands.json: GNUmakefile CMakeLists.txt
 	cmake --preset Release --log-level=VERBOSE -D BOOST_USE_MODULES=ON -D CMAKE_CXX_MODULE_STD=ON \
 	-D CMAKE_CXX_STDLIB_MODULES_JSON=${CMAKE_CXX_STDLIB_MODULES_JSON}
diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index 51cbe6f8..a4d4b1d3 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -14,7 +14,7 @@ if(PROJECT_IS_TOP_LEVEL)
     set(CMAKE_CXX_EXTENSIONS OFF)
     set(CMAKE_CXX_STANDARD_REQUIRED ON)
     if(NOT DEFINED CMAKE_CXX_STANDARD)
-        set(CMAKE_CXX_STANDARD 23)
+        set(CMAKE_CXX_STANDARD 20)
     endif()
     if(NOT DEFINED CMAKE_PREFIX_PATH)
         list(APPEND CMAKE_PREFIX_PATH ../stagedir)

From 04a32b4d8739b04c59173b8144d49867bfec56b3 Mon Sep 17 00:00:00 2001
From: ClausKlein <claus.klein@arcormail.de>
Date: Tue, 23 Jun 2026 07:18:53 +0200
Subject: [PATCH 08/14] pre-commit autoupdate

---
 .pre-commit-config.yaml | 5 +++--
 examples/CMakeLists.txt | 6 +-----
 requirements.txt        | 3 +--
 3 files changed, 5 insertions(+), 9 deletions(-)

diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index 10b94ece..f1bf54f9 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -5,7 +5,8 @@ repos:
   - repo: https://github.com/pre-commit/pre-commit-hooks
     rev: v6.0.0
     hooks:
-      # - id: trailing-whitespace
+      - id: trailing-whitespace
+        exclude: ^patches/\.*(patch|diff)$
       - id: end-of-file-fixer
       - id: check-json
       - id: check-yaml
@@ -23,7 +24,7 @@ repos:
 
   # CMake linting and formatting
   - repo: https://github.com/BlankSpruce/gersemi
-    rev: 0.26.0
+    rev: 0.27.7
     hooks:
       - id: gersemi
         name: CMake linting
diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index a4d4b1d3..97b619e3 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -51,11 +51,7 @@ if(PROJECT_IS_TOP_LEVEL)
             SYSTEM
             FIND_PACKAGE_ARGS
                 1.90.0.1
-                COMPONENTS
-                any
-                asio
-                regex
-                unit_test_framework
+                COMPONENTS any asio regex unit_test_framework
         )
         FetchContent_MakeAvailable(Boost)
     endblock()
diff --git a/requirements.txt b/requirements.txt
index 09fad138..7c02bba1 100644
--- a/requirements.txt
+++ b/requirements.txt
@@ -17,10 +17,9 @@ pre-commit
 ### cmake build context
 bump2version>=1.0.1
 check-jsonschema>=0.31.2
-# cmake-format>=0.6.13
 cmake>=3.30
 codespell>=2.4.2
-conan>=2.13.0
+conan>=2.28.1
 gcovr>=8.6
 ninja>=1.13
 yamllint>=1.35

From 5e0569cdbae52a51d884e130df7e5032bedf34c9 Mon Sep 17 00:00:00 2001
From: ClausKlein <claus.klein@arcormail.de>
Date: Tue, 23 Jun 2026 07:25:43 +0200
Subject: [PATCH 09/14] disable gersemi at pre-commit hooks

---
 .pre-commit-config.yaml | 14 +++++++-------
 1 file changed, 7 insertions(+), 7 deletions(-)

diff --git a/.pre-commit-config.yaml b/.pre-commit-config.yaml
index f1bf54f9..5addb098 100644
--- a/.pre-commit-config.yaml
+++ b/.pre-commit-config.yaml
@@ -6,7 +6,7 @@ repos:
     rev: v6.0.0
     hooks:
       - id: trailing-whitespace
-        exclude: ^patches/\.*(patch|diff)$
+        exclude: ^.*\.(patch|cfg)$
       - id: end-of-file-fixer
       - id: check-json
       - id: check-yaml
@@ -22,12 +22,12 @@ repos:
         types_or: [c++, c, json]
         exclude: docs/TODO.json
 
-  # CMake linting and formatting
-  - repo: https://github.com/BlankSpruce/gersemi
-    rev: 0.27.7
-    hooks:
-      - id: gersemi
-        name: CMake linting
+  # # CMake linting and formatting
+  # - repo: https://github.com/BlankSpruce/gersemi
+  #   rev: 0.27.7
+  #   hooks:
+  #     - id: gersemi
+  #       name: CMake linting
 
   # TODO: Markdown linting
   # Config file: .markdownlint.yaml

From d0636edd25d7847d20c142f7a9d22f2f45b4f0e3 Mon Sep 17 00:00:00 2001
From: ClausKlein <claus.klein@arcormail.de>
Date: Tue, 23 Jun 2026 13:21:49 +0200
Subject: [PATCH 10/14] Patch pfr, lexical_cast, and type_index too

---
 patches/1.91.0/cmake-01.patch | 134 ++++++++++++++++++++++++++++++++++
 1 file changed, 134 insertions(+)

diff --git a/patches/1.91.0/cmake-01.patch b/patches/1.91.0/cmake-01.patch
index 93fc4a66..f0758966 100644
--- a/patches/1.91.0/cmake-01.patch
+++ b/patches/1.91.0/cmake-01.patch
@@ -109,3 +109,137 @@
      endif()
      set(__scope PUBLIC)
  else()
+--- libs/lexical_cast/CMakeLists.txt.old	2026-06-23 12:16:39.262084782 +0200
++++ libs/lexical_cast/CMakeLists.txt	2026-06-23 12:33:54.908892502 +0200
+@@ -3,25 +3,30 @@
+ # Distributed under the Boost Software License, Version 1.0.
+ # https://www.boost.org/LICENSE_1_0.txt
+ 
+-cmake_minimum_required(VERSION 3.5...4.0)
++cmake_minimum_required(VERSION 3.28...4.0)
+ 
+ project(boost_lexical_cast VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)
+ 
+-
+-if (BOOST_USE_MODULES)
++if(BOOST_USE_MODULES)
+     add_library(boost_lexical_cast)
+     target_sources(boost_lexical_cast PUBLIC
+-        FILE_SET modules_public TYPE CXX_MODULES FILES
+-            ${CMAKE_CURRENT_LIST_DIR}/modules/boost_lexical_cast.cppm
++        FILE_SET modules_public
++            TYPE CXX_MODULES
++            FILES modules/boost_lexical_cast.cppm
+     )
+ 
+     target_compile_features(boost_lexical_cast PUBLIC cxx_std_20)
+     target_compile_definitions(boost_lexical_cast PUBLIC BOOST_USE_MODULES)
+-    if (CMAKE_CXX_COMPILER_IMPORT_STD)
+-        target_compile_definitions(boost_lexical_cast PRIVATE BOOST_LEXICAL_CAST_USE_STD_MODULE)
++    set(__boost_cxx_standard ${CMAKE_CXX_STANDARD})
++    if(NOT __boost_cxx_standard)
++        set(__boost_cxx_standard 23)
++    endif()
++    if(CMAKE_CXX_COMPILER_IMPORT_STD AND ${__boost_cxx_standard} IN_LIST CMAKE_CXX_COMPILER_IMPORT_STD)
++        target_compile_definitions(boost_lexical_cast PUBLIC BOOST_LEXICAL_CAST_USE_STD_MODULE)
++        target_compile_features(boost_lexical_cast PUBLIC cxx_std_${__boost_cxx_standard})
+         message(STATUS "Using `import std;`")
+     else()
+-        message(STATUS "`import std;` is not awailable")
++        message(STATUS "`import std;` is not available")
+     endif()
+     set(__scope PUBLIC)
+ else()
+@@ -43,4 +48,3 @@
+ if(BUILD_TESTING)
+   add_subdirectory(test)
+ endif()
+-
+--- libs/pfr/CMakeLists.txt.old	2026-06-23 12:57:13.166120620 +0200
++++ libs/pfr/CMakeLists.txt	2026-06-23 13:06:24.695676463 +0200
+@@ -4,20 +4,27 @@
+ # Distributed under the Boost Software License, Version 1.0.
+ # https://www.boost.org/LICENSE_1_0.txt
+ 
+-cmake_minimum_required(VERSION 3.5...3.31)
++cmake_minimum_required(VERSION 3.28...4.0)
+ 
+ project(boost_pfr VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)
+ 
+-if (BOOST_USE_MODULES)
++if(BOOST_USE_MODULES)
+     add_library(boost_pfr)
+     target_sources(boost_pfr PUBLIC
+-        FILE_SET modules_public TYPE CXX_MODULES FILES modules/boost_pfr.cppm
++        FILE_SET modules_public
++            TYPE CXX_MODULES
++            FILES modules/boost_pfr.cppm
+     )
+ 
+     target_compile_features(boost_pfr PUBLIC cxx_std_20)
+     target_compile_definitions(boost_pfr PUBLIC BOOST_USE_MODULES)
+-    if (CMAKE_CXX_COMPILER_IMPORT_STD)
+-        target_compile_definitions(boost_pfr PRIVATE BOOST_PFR_USE_STD_MODULE)
++    set(__boost_cxx_standard ${CMAKE_CXX_STANDARD})
++    if(NOT __boost_cxx_standard)
++        set(__boost_cxx_standard 23)
++    endif()
++    if(CMAKE_CXX_COMPILER_IMPORT_STD AND ${__boost_cxx_standard} IN_LIST CMAKE_CXX_COMPILER_IMPORT_STD)
++        target_compile_definitions(boost_pfr PUBLIC BOOST_PFR_USE_STD_MODULE)
++        target_compile_features(boost_pfr PUBLIC cxx_std_${__boost_cxx_standard})
+         message(STATUS "Using `import std;`")
+     else()
+         message(STATUS "`import std;` is not available")
+@@ -31,11 +38,11 @@
+ add_library(Boost::pfr ALIAS boost_pfr)
+ 
+ enable_testing()
+-if (BUILD_TESTING AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/test/CMakeLists.txt")
++if(BUILD_TESTING AND EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/test/CMakeLists.txt")
+     add_subdirectory(test)
+ endif()
+ 
+-if (BOOST_USE_MODULES AND BUILD_TESTING)
++if(BOOST_USE_MODULES AND BUILD_TESTING)
+     add_executable(boost_pfr_module_usage modules/usage_sample.cpp)
+     target_link_libraries(boost_pfr_module_usage PRIVATE Boost::pfr)
+     add_test(NAME boost_pfr_module_usage COMMAND boost_pfr_module_usage)
+--- libs/type_index/CMakeLists.txt.old	2026-06-23 12:37:11.660738834 +0200
++++ libs/type_index/CMakeLists.txt	2026-06-23 12:44:32.000000000 +0200
+@@ -3,23 +3,30 @@
+ # Distributed under the Boost Software License, Version 1.0.
+ # https://www.boost.org/LICENSE_1_0.txt
+ 
+-cmake_minimum_required(VERSION 3.5...3.31)
++cmake_minimum_required(VERSION 3.28...4.0)
+ 
+ project(boost_type_index VERSION "${BOOST_SUPERPROJECT_VERSION}" LANGUAGES CXX)
+ 
+-if (BOOST_USE_MODULES)
++if(BOOST_USE_MODULES)
+     add_library(boost_type_index)
+     target_sources(boost_type_index PUBLIC
+-        FILE_SET modules_public TYPE CXX_MODULES FILES modules/boost_type_index.cppm
++        FILE_SET modules_public
++            TYPE CXX_MODULES
++            FILES modules/boost_type_index.cppm
+     )
+ 
+     target_compile_features(boost_type_index PUBLIC cxx_std_20)
+     target_compile_definitions(boost_type_index PUBLIC BOOST_USE_MODULES)
+-    if (CMAKE_CXX_COMPILER_IMPORT_STD)
+-        target_compile_definitions(boost_type_index PRIVATE BOOST_TYPE_INDEX_USE_STD_MODULE)
++    set(__boost_cxx_standard ${CMAKE_CXX_STANDARD})
++    if(NOT __boost_cxx_standard)
++        set(__boost_cxx_standard 23)
++    endif()
++    if(CMAKE_CXX_COMPILER_IMPORT_STD AND ${__boost_cxx_standard} IN_LIST CMAKE_CXX_COMPILER_IMPORT_STD)
++        target_compile_definitions(boost_type_index PUBLIC BOOST_TYPE_INDEX_USE_STD_MODULE)
++        target_compile_features(boost_type_index PUBLIC cxx_std_${__boost_cxx_standard})
+         message(STATUS "Using `import std;`")
+     else()
+-        message(STATUS "`import std;` is not awailable")
++        message(STATUS "`import std;` is not available")
+     endif()
+     set(__scope PUBLIC)
+ else()

From d7d56594b7a01a71842547a051a681f777da1334 Mon Sep 17 00:00:00 2001
From: ClausKlein <claus.klein@arcormail.de>
Date: Tue, 23 Jun 2026 14:20:18 +0200
Subject: [PATCH 11/14] Prepare manual test with g++-16 on OSX

---
 CMakeLists.txt                             | 47 ++++++++--------------
 GNUmakefile                                | 22 +++++-----
 cmake/enable-experimental-import-std.cmake | 24 +++++++++++
 examples/CMakeLists.txt                    | 10 ++++-
 4 files changed, 61 insertions(+), 42 deletions(-)
 create mode 100644 cmake/enable-experimental-import-std.cmake

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 0ee54adf..04405bfe 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -1,28 +1,6 @@
 cmake_minimum_required(VERSION 3.30...4.4)
 
-if(CMAKE_VERSION VERSION_GREATER_EQUAL "3.30.0")
-    if(CMAKE_VERSION VERSION_LESS "3.31.8")
-        set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
-            "0e5b6991-d74f-4b3d-a41c-cf096e0b2508"
-        )
-    elseif(CMAKE_VERSION VERSION_LESS "4.0.0")
-        set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
-            "d0edc3af-4c50-42ea-a356-e2862fe7a444"
-        )
-    elseif(CMAKE_VERSION VERSION_LESS "4.0.3")
-        set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
-            "a9e1cf81-9932-4810-974b-6eccaf14e457"
-        )
-    elseif(CMAKE_VERSION VERSION_LESS "4.3.0")
-        set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
-            "d0edc3af-4c50-42ea-a356-e2862fe7a444"
-        )
-    elseif(CMAKE_VERSION VERSION_LESS "4.4.0")
-        set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
-            "451f2fe2-a8a2-47c3-bc32-94786d8fc91b"
-        )
-    endif()
-endif()
+include(cmake/enable-experimental-import-std.cmake)
 
 # gersemi: off
 project(Boost-CMake LANGUAGES CXX VERSION 1.91.0.1)
@@ -36,7 +14,11 @@ if(PROJECT_IS_TOP_LEVEL)
     option(BOOST_USE_MODULES "Do build with CXX_MODULES if possible" OFF)
     if(BOOST_USE_MODULES)
         set(CMAKE_CXX_SCAN_FOR_MODULES ON)
-        set(CMAKE_CXX_MODULE_STD OFF) # TODO(CK): not yet!
+        if(CMAKE_CXX_STANDARD GREATER_EQUAL 23)
+            set(CMAKE_CXX_MODULE_STD ON)
+        else()
+            set(CMAKE_CXX_MODULE_STD OFF)
+        endif()
     else()
         set(CMAKE_CXX_SCAN_FOR_MODULES OFF)
     endif()
@@ -98,11 +80,16 @@ endif()
 set(BOOST_LIBS_REQUIRED headers)
 set(BOOST_EXCLUDE_LIBRARIES
     cobalt
+    contract # FIXME: deps on any! CK
     fiber
     iostreams
+    graph # FIXME: deps on any! CK
     locale
     log
     json
+    program_options # FIXME: deps on any! CK
+    property_map # FIXME: deps on any! CK
+    property_tree # FIXME: deps on any! CK
     timer
     wave
 )
@@ -118,22 +105,22 @@ if(PROJECT_IS_TOP_LEVEL AND NOT BOOST_BUILD_ALL)
         beast # deps on type_index
         chrono
         container
-        contract # deps on any
+        contract # TODO: deps on any! CK
         coroutine
         exception
         filesystem # FIXME: Warnings with g++-15 on OSX! CK
         geometry # deps on any
-        graph # deps on any, serialization
+        graph # TODO: deps on any, serialization! CK
         json
-        lexical_cast # TODO(CK): use BOOST_USE_MODULES but import std wrong!
+        lexical_cast # use BOOST_USE_MODULES
         math
         optional
         parser # deps on type_index
         pfr # use BOOST_USE_MODULES
         process # deps on type_index
-        program_options # deps on any
-        property_map # deps on any
-        property_tree # deps on any
+        program_options # TODO: deps on any! CK
+        property_map # TODO: deps on any! CK
+        property_tree # TODO: deps on any! CK
         regex
         serialization
         signals2
diff --git a/GNUmakefile b/GNUmakefile
index 872024de..c0bd8416 100644
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -12,19 +12,19 @@ ifeq (${hostSystemName},Darwin)
   export LLVM_DIR:=$(shell realpath ${LLVM_PREFIX})
   export PATH:=${LLVM_DIR}/bin:${PATH}
 
-  export CMAKE_CXX_STDLIB_MODULES_JSON:=${LLVM_DIR}/lib/c++/libc++.modules.json
-  export CXX:=clang++
-  export LDFLAGS:=-L$(LLVM_DIR)/lib/c++ -lc++abi # NO! -lc++ -lc++experimental
-  export GCOV:="llvm-cov gcov"
+  # export CMAKE_CXX_STDLIB_MODULES_JSON:=${LLVM_DIR}/lib/c++/libc++.modules.json
+  # export CXX:=clang++
+  # export LDFLAGS:=-L$(LLVM_DIR)/lib/c++ -lc++abi # NO! -lc++ -lc++experimental
+  # export GCOV:="llvm-cov gcov"
 
-  ### TODO: to test g++-15:
+  ### TODO: to test g++-16:
   export GCC_PREFIX:=$(shell brew --prefix gcc)
   export GCC_DIR:=$(shell realpath ${GCC_PREFIX})
 
-  # export CMAKE_CXX_STDLIB_MODULES_JSON:=${GCC_DIR}/lib/gcc/current/libstdc++.modules.json
-  # export CXX:=g++-15
-  # export CXXFLAGS:=-stdlib=libstdc++
-  # export GCOV:="gcov"
+  export CMAKE_CXX_STDLIB_MODULES_JSON:=${GCC_DIR}/lib/gcc/current/libstdc++.modules.json
+  export CXX:=g++-16
+  export CXXFLAGS:=-stdlib=libstdc++
+  export GCOV:="gcov"
 else ifeq (${hostSystemName},Linux)
   export LLVM_DIR:=/usr/lib/llvm-20
   export PATH:=${LLVM_DIR}/bin:${PATH}
@@ -50,7 +50,7 @@ compile_commands.json: build/Release/compile_commands.json
 # NOTE: Works only yet on OSX with clang v22.1.7 with this arguments! CK
 build/Release/compile_commands.json: GNUmakefile CMakeLists.txt
 	cmake --preset Release --log-level=VERBOSE -D BOOST_USE_MODULES=ON -D CMAKE_CXX_MODULE_STD=ON \
-	-D CMAKE_CXX_STDLIB_MODULES_JSON=${CMAKE_CXX_STDLIB_MODULES_JSON}
+	-D CMAKE_CXX_STDLIB_MODULES_JSON=${CMAKE_CXX_STDLIB_MODULES_JSON} -D CMAKE_CXX_STANDARD=26
 
 build: compile_commands.json
 	cmake --build --preset Release
@@ -66,7 +66,7 @@ clean:
 	-find . -name '*~' -delete
 
 distclean: # XXX clean
-	rm -rf build stagedir
+	rm -rf build stagedir .cache compile_commands.json
 
 format:
 	git ls-files ::*.cmake ::*CMakeLists.txt | xargs gersemi -i
diff --git a/cmake/enable-experimental-import-std.cmake b/cmake/enable-experimental-import-std.cmake
new file mode 100644
index 00000000..e41a5f7c
--- /dev/null
+++ b/cmake/enable-experimental-import-std.cmake
@@ -0,0 +1,24 @@
+# SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+if(CMAKE_VERSION VERSION_GREATER_EQUAL "4.3.0")
+    set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
+        "451f2fe2-a8a2-47c3-bc32-94786d8fc91b"
+    )
+elseif(CMAKE_VERSION VERSION_GREATER_EQUAL "3.30.0")
+    if(CMAKE_VERSION VERSION_LESS "3.31.8")
+        set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
+            "0e5b6991-d74f-4b3d-a41c-cf096e0b2508"
+        )
+    elseif(CMAKE_VERSION VERSION_LESS "4.0.0")
+        set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
+            "d0edc3af-4c50-42ea-a356-e2862fe7a444"
+        )
+    elseif(CMAKE_VERSION VERSION_LESS "4.0.3")
+        set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
+            "a9e1cf81-9932-4810-974b-6eccaf14e457"
+        )
+    elseif(CMAKE_VERSION VERSION_LESS "4.3.0")
+        set(CMAKE_EXPERIMENTAL_CXX_IMPORT_STD
+            "d0edc3af-4c50-42ea-a356-e2862fe7a444"
+        )
+    endif()
+endif()
diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index 97b619e3..eb717f1b 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -1,5 +1,7 @@
 cmake_minimum_required(VERSION 3.30...4.4)
 
+include(../cmake/enable-experimental-import-std.cmake)
+
 # test boost-cmake v1.91.0-rc1
 project(BoostExamples LANGUAGES CXX)
 
@@ -24,7 +26,11 @@ if(PROJECT_IS_TOP_LEVEL)
     endif()
     if(BOOST_USE_MODULES)
         set(CMAKE_CXX_SCAN_FOR_MODULES ON)
-        set(CMAKE_CXX_MODULE_STD OFF) # TODO(CK): not yet!
+        if(CMAKE_CXX_STANDARD GREATER_EQUAL 23)
+            set(CMAKE_CXX_MODULE_STD ON)
+        else()
+            set(CMAKE_CXX_MODULE_STD OFF)
+        endif()
     else()
         set(CMAKE_CXX_SCAN_FOR_MODULES OFF)
     endif()
@@ -94,7 +100,9 @@ if(NOT PROJECT_IS_TOP_LEVEL)
             "${CMAKE_CURRENT_BINARY_DIR}/find-package-test" --build-generator
             ${CMAKE_GENERATOR} --build-makeprogram ${CMAKE_MAKE_PROGRAM}
             --build-options "-DCMAKE_CXX_COMPILER=${CMAKE_CXX_COMPILER}"
+            "-DCMAKE_CXX_MODULE_STD=${CMAKE_CXX_MODULE_STD}"
             "-DCMAKE_CXX_STANDARD=${CMAKE_CXX_STANDARD}"
+            "-DCMAKE_CXX_STDLIB_MODULES_JSON=${CMAKE_CXX_STDLIB_MODULES_JSON}"
             "-DCMAKE_BUILD_TYPE=$<CONFIG>"
             "-DCMAKE_PREFIX_PATH=${CMAKE_PREFIX_PATH}"
             "-DBOOST_USE_MODULES=${BOOST_USE_MODULES}"

From f5bd1e5c065505a303d1fd9c2826b82718804461 Mon Sep 17 00:00:00 2001
From: ClausKlein <claus.klein@arcormail.de>
Date: Tue, 23 Jun 2026 17:32:00 +0200
Subject: [PATCH 12/14] Disable Boost::any for now, caused by Windows build
 problems!

---
 CMakeLists.txt          | 1 +
 examples/CMakeLists.txt | 4 ++--
 2 files changed, 3 insertions(+), 2 deletions(-)

diff --git a/CMakeLists.txt b/CMakeLists.txt
index 04405bfe..6397ae21 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -79,6 +79,7 @@ endif()
 
 set(BOOST_LIBS_REQUIRED headers)
 set(BOOST_EXCLUDE_LIBRARIES
+    any
     cobalt
     contract # FIXME: deps on any! CK
     fiber
diff --git a/examples/CMakeLists.txt b/examples/CMakeLists.txt
index eb717f1b..2064d72d 100644
--- a/examples/CMakeLists.txt
+++ b/examples/CMakeLists.txt
@@ -38,7 +38,7 @@ if(PROJECT_IS_TOP_LEVEL)
     block()
         set(BOOST_DEBUG YES)
         set(BOOST_VERBOSE YES)
-        list(APPEND BOOST_INCLUDE_LIBRARIES any asio regex test)
+        list(APPEND BOOST_INCLUDE_LIBRARIES asio regex test)
 
         set(CMAKE_SKIP_INSTALL_RULES ON)
 
@@ -65,7 +65,7 @@ endif()
 
 # ---- Create test binaries ----
 
-if(BOOST_USE_MODULES)
+if(BOOST_USE_MODULES AND TARGET Boost::any)
     add_executable(ExampleModuleAny any_module_usage_sample.cpp)
     target_link_libraries(ExampleModuleAny PRIVATE Boost::any)
     add_test(NAME ExampleModuleAny COMMAND ExampleModuleAny)

From e9e83486d0514b42211903da8582d26dfc800831 Mon Sep 17 00:00:00 2001
From: ClausKlein <claus.klein@arcormail.de>
Date: Tue, 23 Jun 2026 19:42:16 +0200
Subject: [PATCH 13/14] Upgrade Node.js CI actions as 20 is deprecated

---
 .github/workflows/cmake.yml | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/.github/workflows/cmake.yml b/.github/workflows/cmake.yml
index 22331fd4..c793cedd 100644
--- a/.github/workflows/cmake.yml
+++ b/.github/workflows/cmake.yml
@@ -36,12 +36,12 @@ jobs:
       - name: Setup build environment
         uses: lukka/get-cmake@latest
         with:
-          cmakeVersion: "~4.3.4"
-          ninjaVersion: "^1.13.0"
+          cmakeVersion: "^4.3.4"
+          ninjaVersion: "~1.13.0"
 
       - name: Setup MSVC
         if: startsWith(matrix.os, 'windows')
-        uses: TheMrMilchmann/setup-msvc-dev@v3
+        uses: TheMrMilchmann/setup-msvc-dev@v4
         with:
           arch: x64
 

From b9dcf642baa8a08b3f398b4cab80a0007d062572 Mon Sep 17 00:00:00 2001
From: ClausKlein <claus.klein@arcormail.de>
Date: Tue, 23 Jun 2026 19:49:27 +0200
Subject: [PATCH 14/14] Use llvm-22 on CI

---
 .github/workflows/cmake.yml | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/.github/workflows/cmake.yml b/.github/workflows/cmake.yml
index c793cedd..c49aa250 100644
--- a/.github/workflows/cmake.yml
+++ b/.github/workflows/cmake.yml
@@ -27,9 +27,9 @@ jobs:
       fail-fast: false
 
       matrix:
-        os: [macos, ubuntu, windows]
+        os: [macos-latest, ubuntu-26.04, windows-latest]
 
-    runs-on: ${{ matrix.os }}-latest
+    runs-on: ${{ matrix.os }} # -latest
 
     steps:
       - uses: actions/checkout@v4
@@ -46,10 +46,10 @@ jobs:
           arch: x64
 
       - name: Setup Cpp
-        if: matrix.os != 'windows'
+        if: matrix.os != 'windows-latest'
         uses: aminya/setup-cpp@v1
         with:
-          compiler: llvm
+          compiler: llvm-22
 
       - name: Configure CMake
         run: cmake --preset ${{env.BUILD_TYPE}} --log-level=VERBOSE -D BOOST_USE_MODULES=ON