Add infrastructure for C++26 compiler support (GCC16)

.github/workflows/cmake.yml

@@ -211,6 +211,18 @@ jobs:
                   std: 20,
                 },
             }
+          - {
+              name: "Ubuntu GCC-16 (C++26 Reflection)",
+              os: ubuntu-24.04,
+              compiler:
+                {
+                  type: GCC16,
+                  version: 16,
+                  cc: "gcc-16",
+                  cxx: "g++-16",
+                  std: 26,
+                },
+            }
     steps:
       - uses: actions/checkout@v4
       - uses: lukka/[email protected]
@@ -231,9 +243,15 @@ jobs:
         with:
           version: ${{ matrix.settings.compiler.version }}
           platform: x64
+      - name: Install GCC 16 from ubuntu-toolchain-r PPA
+        if: matrix.settings.compiler.type == 'GCC16'
+        run: |
+          sudo add-apt-repository -y ppa:ubuntu-toolchain-r/test
+          sudo apt-get update
+          sudo apt-get install -y --no-install-recommends g++-16 gcc-16
       - name: Install uv
         uses: astral-sh/setup-uv@v5
       - name: Set up Python
         run: uv sync
       - name: Run CMake
-        run: ./scripts/cmake.sh --${{ matrix.configuration == 'Debug' && 'debug' || 'release' }}
+        run: ./scripts/cmake.sh --${{ matrix.configuration == 'Debug' && 'debug' || 'release' }} ${{ matrix.settings.compiler.type == 'GCC16' && '--reflection' || '' }}

CMakeLists.txt

@@ -1,4 +1,4 @@
-cmake_minimum_required(VERSION 3.22.4 FATAL_ERROR)
+cmake_minimum_required(VERSION 3.25 FATAL_ERROR)
 cmake_policy(SET CMP0127 NEW)
 cmake_policy(SET CMP0135 NEW)
 cmake_policy(SET CMP0140 NEW)
@@ -29,6 +29,35 @@ option(ENABLE_UBSAN "Enable Undefined Behaviour Sanitizer" OFF)
 option(ENABLE_TSAN "Enable Thread Sanitizer" OFF)
 option(ENABLE_MSAN "Enable Memory Sanitizer" OFF)
 
+option(
+  XYZ_PROTOCOL_BUILD_REFLECTION
+  "Also build and test tutorials/reflection.cc, which requires a compiler \
+with C++26 P2996 reflection support (eg. GCC 16+ with -freflection)."
+  OFF)
+
+if(XYZ_PROTOCOL_BUILD_REFLECTION)
+  set(CMAKE_CXX_STANDARD 26)
+  if(CMAKE_CXX_COMPILER_ID STREQUAL "GNU")
+    set(CMAKE_REQUIRED_FLAGS "-freflection")
+  endif()
+  include(CheckCXXSourceCompiles)
+  check_cxx_source_compiles(
+    "
+    #include <meta>
+    constexpr std::meta::info reflection_of_int = ^^int;
+    int main() {}
+    "
+    XYZ_PROTOCOL_REFLECTION_SUPPORTED)
+  unset(CMAKE_REQUIRED_FLAGS)
+  if(NOT XYZ_PROTOCOL_REFLECTION_SUPPORTED)
+    message(
+      FATAL_ERROR
+        "XYZ_PROTOCOL_BUILD_REFLECTION is ON but the compiler "
+        "(${CMAKE_CXX_COMPILER_ID} ${CMAKE_CXX_COMPILER_VERSION}) does not "
+        "support C++26 reflection.")
+  endif()
+endif()
+
 if(ENABLE_ASAN OR ENABLE_UBSAN OR ENABLE_TSAN OR ENABLE_MSAN)
   set(ENABLE_SANITIZERS ON)
 endif()

cmake/xyz_add_test.cmake

@@ -50,7 +50,7 @@ function(xyz_add_test)
   if(NOT XYZ_VERSION)
     set(XYZ_VERSION 20)
   else()
-    set(VALID_TARGET_VERSIONS 11 14 17 20 23)
+    set(VALID_TARGET_VERSIONS 11 14 17 20 23 26)
     list(FIND VALID_TARGET_VERSIONS ${XYZ_VERSION} index)
     if(index EQUAL -1)
       message(FATAL_ERROR "TYPE must be one of <${VALID_TARGET_VERSIONS}>")

docker/Dockerfile

@@ -1,8 +1,8 @@
+# syntax=docker/dockerfile:1
 FROM ubuntu:24.04
 
 # Install base tools and C++ development tools.
 RUN apt-get update && apt-get install -y --no-install-recommends \
-    cmake \
     curl \
     g++ \
     gdb \
@@ -18,13 +18,20 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     ssh \
     unzip \
     wget \
-&& rm -rf /var/lib/apt/lists/*
+    && rm -rf /var/lib/apt/lists/*
 
 # Install newer CMake from kitware.
 RUN wget -O - https://apt.kitware.com/keys/kitware-archive-latest.asc 2>/dev/null | gpg --dearmor - | tee /etc/apt/trusted.gpg.d/kitware.gpg >/dev/null \
-&& apt-add-repository -y 'deb https://apt.kitware.com/ubuntu/ noble main' \
-&& apt-get update && apt-get install -y --no-install-recommends cmake \
-&& rm -rf /var/lib/apt/lists/*
+    && apt-add-repository -y 'deb https://apt.kitware.com/ubuntu/ noble main' \
+    && apt-get update && apt-get install -y --no-install-recommends cmake \
+    && rm -rf /var/lib/apt/lists/*
+
+# Install GCC 16 (experimental C++26 reflection support via -std=c++26
+# -freflection, see https://gcc.gnu.org/gcc-16/changes.html) from the Ubuntu
+# Toolchain Test PPA, since Ubuntu 24.04's own repos only go up to GCC 14.
+RUN add-apt-repository -y ppa:ubuntu-toolchain-r/test \
+    && apt-get update && apt-get install -y --no-install-recommends g++-16 gcc-16 \
+    && rm -rf /var/lib/apt/lists/*
 
 # Install bazelisk.
 RUN ARCH=$(dpkg --print-architecture) && \
@@ -36,11 +43,11 @@ RUN ARCH=$(dpkg --print-architecture) && \
 # https://github.com/cli/cli/blob/trunk/docs/install_linux.md (not reliably
 # available via Ubuntu 24.04's default repos).
 RUN mkdir -p -m 755 /etc/apt/keyrings \
-&& wget -nv -O /etc/apt/keyrings/githubcli-archive-keyring.gpg https://cli.github.com/packages/githubcli-archive-keyring.gpg \
-&& chmod go+r /etc/apt/keyrings/githubcli-archive-keyring.gpg \
-&& echo "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/githubcli-archive-keyring.gpg] https://cli.github.com/packages stable main" | tee /etc/apt/sources.list.d/github-cli.list >/dev/null \
-&& apt-get update && apt-get install -y --no-install-recommends gh \
-&& rm -rf /var/lib/apt/lists/*
+    && wget -nv -O /etc/apt/keyrings/githubcli-archive-keyring.gpg https://cli.github.com/packages/githubcli-archive-keyring.gpg \
+    && chmod go+r /etc/apt/keyrings/githubcli-archive-keyring.gpg \
+    && echo "deb [arch=$(dpkg --print-architecture) signed-by=/etc/apt/keyrings/githubcli-archive-keyring.gpg] https://cli.github.com/packages stable main" | tee /etc/apt/sources.list.d/github-cli.list >/dev/null \
+    && apt-get update && apt-get install -y --no-install-recommends gh \
+    && rm -rf /var/lib/apt/lists/*
 
 # Install jj (Jujutsu VCS) from its GitHub release binary. The jujutsu apt
 # package isn't built for Ubuntu 24.04 (noble) or Debian stable; it only