Allow selection of system default C++ version.

This leaves the decision of which C++ version to use up to our users.  We still have a static_assert in port_def.inc that will prevent pre-C++14 usage.

PiperOrigin-RevId: 501351066

CMakeLists.txt

@@ -40,27 +40,6 @@ if(protobuf_DEPRECATED_CMAKE_SUBDIRECTORY_USAGE)
   get_filename_component(protobuf_SOURCE_DIR ${protobuf_SOURCE_DIR} DIRECTORY)
 endif()
 
-# Add C++14 flags
-if(NOT CMAKE_CXX_STANDARD)
-  set(CMAKE_CXX_STANDARD 14)
-endif()
-if(CYGWIN)
-  string(REGEX_MATCH "-std=gnu\\+\\+([0-9]+)" _protobuf_CXX_STD "${CMAKE_CXX_FLAGS}")
-endif()
-if(NOT _protobuf_CXX_STD)
-  set(_protobuf_CXX_STD "${CMAKE_CXX_STANDARD}")
-endif()
-if(_protobuf_CXX_STD LESS "14")
-  message(FATAL_ERROR "Protocol Buffers requires at least C++14, but is configured for C++${_protobuf_CXX_STD}")
-endif()
-if(CYGWIN)
-  set(CMAKE_CXX_FLAGS "${CMAKE_CXX_FLAGS} -std=gnu++${_protobuf_CXX_STD}")
-else()
-  set(CMAKE_CXX_STANDARD ${_protobuf_CXX_STD})
-  set(CMAKE_CXX_STANDARD_REQUIRED ON)
-  set(CMAKE_CXX_EXTENSIONS OFF)
-endif()
-
 # The Intel compiler isn't able to deal with noinline member functions of
 # template classes defined in headers.  As such it spams the output with
 #   warning #2196: routine is both "inline" and "noinline"

cmake/README.md

@@ -8,6 +8,19 @@ currently support CMake 3.5 and newer on both [Windows](#windows-builds) and
 Most of the instructions will be given using CMake's command-line interface, but
 the same actions can be performed using appropriate GUI tools.
 
+# CMake Flags
+
+## C++ Version
+
+By default, CMake will use whatever C++ version is the system default.  Since
+protobuf requires C++14 or newer, sometimes you will need to explicitly override
+this.  For example, the following:
+
+    cmake . -DCMAKE_CXX_STANDARD=14
+    cmake --build
+
+will build protobuf using C++14 (see [CXX_STANDARD](https://cmake.org/cmake/help/latest/prop_tgt/CXX_STANDARD.html#prop_tgt:CXX_STANDARD){.external} for all available options).
+
 # Windows Builds
 
 On Windows, you can build the project from *Command Prompt* and using an

kokoro/common/cmake.sh

@@ -60,7 +60,8 @@ mkdir -p "${BUILD_DIR}"
   caplog 01_configure \
     cmake -S "${SOURCE_DIR}" \
     ${CMAKE_BUILD_TYPE_FLAG} \
-    ${CAPLOG_CMAKE_ARGS:-}
+    ${CAPLOG_CMAKE_ARGS:-} \
+    -DCMAKE_CXX_STANDARD=14
 )
 if [[ -n ${CAPLOG_DIR:-} ]]; then
   # Save configuration logs.

kokoro/linux/32-bit/test_php.sh

@@ -38,7 +38,7 @@ build_php_c() {
 
 mkdir -p build
 pushd build
-cmake ..
+cmake .. -DCMAKE_CXX_STANDARD=14
 cmake --build . -- -j20
 ctest --verbose --parallel 20
 export PROTOC=$(pwd)/protoc

kokoro/linux/cmake/build.sh

@@ -8,7 +8,7 @@ set -eux
 cd $(dirname $0)/../../..
 GIT_REPO_ROOT=`pwd`
 
-CONTAINER_IMAGE=gcr.io/protobuf-build/cmake/linux@sha256:79e6ed9d7f3f8e56167a3309a521e5b7e6a212bfb19855c65ee1cbb6f9099671
+CONTAINER_IMAGE=gcr.io/protobuf-build/cmake/linux@sha256:bd17c13255c8c7af2821c6597ad96568d714c0ea9d9e27d81b108fcc195ca858
 
 # Update git submodules and regenerate files
 git submodule update --init --recursive
@@ -22,7 +22,10 @@ docker run \
   --cidfile $tmpfile \
   -v $GIT_REPO_ROOT:/workspace \
   $CONTAINER_IMAGE \
-  /test.sh -Dprotobuf_BUILD_CONFORMANCE=ON -Dprotobuf_BUILD_EXAMPLES=ON
+  /test.sh \
+    -Dprotobuf_BUILD_CONFORMANCE=ON \
+    -Dprotobuf_BUILD_EXAMPLES=ON \
+    -DCMAKE_CXX_STANDARD=14
 
 # Save logs for Kokoro
 docker cp \

kokoro/linux/cmake_install/build.sh

@@ -8,7 +8,7 @@ set -eux
 cd $(dirname $0)/../../..
 GIT_REPO_ROOT=`pwd`
 
-CONTAINER_IMAGE=gcr.io/protobuf-build/cmake/linux@sha256:79e6ed9d7f3f8e56167a3309a521e5b7e6a212bfb19855c65ee1cbb6f9099671
+CONTAINER_IMAGE=gcr.io/protobuf-build/cmake/linux@sha256:bd17c13255c8c7af2821c6597ad96568d714c0ea9d9e27d81b108fcc195ca858
 
 # Update git submodules and regenerate files
 git submodule update --init --recursive
@@ -22,10 +22,11 @@ docker run \
   --cidfile $tmpfile \
   -v $GIT_REPO_ROOT:/workspace \
   $CONTAINER_IMAGE \
-  "/install.sh && /test.sh \
-  -Dprotobuf_REMOVE_INSTALLED_HEADERS=ON \
-  -Dprotobuf_BUILD_PROTOBUF_BINARIES=OFF \
-  -Dprotobuf_BUILD_CONFORMANCE=ON"
+  "/install.sh -DCMAKE_CXX_STANDARD=14 && /test.sh \
+    -Dprotobuf_REMOVE_INSTALLED_HEADERS=ON \
+    -Dprotobuf_BUILD_PROTOBUF_BINARIES=OFF \
+    -Dprotobuf_BUILD_CONFORMANCE=ON \
+    -DCMAKE_CXX_STANDARD=14"
 
 
 # Save logs for Kokoro

kokoro/linux/cmake_ninja/build.sh

@@ -8,7 +8,7 @@ set -eux
 cd $(dirname $0)/../../..
 GIT_REPO_ROOT=`pwd`
 
-CONTAINER_IMAGE=gcr.io/protobuf-build/cmake/linux@sha256:79e6ed9d7f3f8e56167a3309a521e5b7e6a212bfb19855c65ee1cbb6f9099671
+CONTAINER_IMAGE=gcr.io/protobuf-build/cmake/linux@sha256:bd17c13255c8c7af2821c6597ad96568d714c0ea9d9e27d81b108fcc195ca858
 
 # Update git submodules and regenerate files
 git submodule update --init --recursive
@@ -22,7 +22,7 @@ docker run \
   --cidfile $tmpfile \
   -v $GIT_REPO_ROOT:/workspace \
   $CONTAINER_IMAGE \
-  /test.sh -G Ninja -Dprotobuf_BUILD_CONFORMANCE=ON
+  /test.sh -G Ninja -Dprotobuf_BUILD_CONFORMANCE=ON -DCMAKE_CXX_STANDARD=14
 
 # Save logs for Kokoro
 docker cp \

kokoro/linux/cmake_shared/build.sh

@@ -8,7 +8,7 @@ set -eux
 cd $(dirname $0)/../../..
 GIT_REPO_ROOT=`pwd`
 
-CONTAINER_IMAGE=gcr.io/protobuf-build/cmake/linux@sha256:79e6ed9d7f3f8e56167a3309a521e5b7e6a212bfb19855c65ee1cbb6f9099671
+CONTAINER_IMAGE=gcr.io/protobuf-build/cmake/linux@sha256:bd17c13255c8c7af2821c6597ad96568d714c0ea9d9e27d81b108fcc195ca858
 
 # Update git submodules and regenerate files
 git submodule update --init --recursive
@@ -22,7 +22,10 @@ docker run \
   --cidfile $tmpfile \
   -v $GIT_REPO_ROOT:/workspace \
   $CONTAINER_IMAGE \
-  /test.sh -Dprotobuf_BUILD_CONFORMANCE=ON -Dprotobuf_BUILD_SHARED_LIBS=ON
+  /test.sh \
+    -Dprotobuf_BUILD_CONFORMANCE=ON \
+    -Dprotobuf_BUILD_SHARED_LIBS=ON \
+    -DCMAKE_CXX_STANDARD=14
 
 # Save logs for Kokoro
 docker cp \