Remove stale references to C++14.

The main branch was upgraded to use C++17 after the 29.0 branch cut, in commit
fe53593

There are still stale references to C++14 in the code, build chain, and
READMEs. This commit cleans up the stale configs and settings.

PiperOrigin-RevId: 706000867

Author: tonyliaoss

.github/workflows/test_cpp.yml

@@ -45,8 +45,7 @@ jobs:
             cache_key: Bazel7
             image: "us-docker.pkg.dev/protobuf-build/containers/common/linux/bazel:7.1.2-27cf7b86212020d7e552bc13b1e084abb971da75"
             targets: "//src/... //third_party/utf8_range/..."
-            # TODO: remove -Wno-unreachable-code" when dropping C++14
-          - config: { name: "Bazel7 with Bzlmod", flags: --enable_bzlmod --enable_workspace --per_file_copt=.*/absl/strings/string_view.h@-Wno-unreachable-code --cxxopt="-Wno-self-assign-overloaded" }
+          - config: { name: "Bazel7 with Bzlmod", flags: --enable_bzlmod --enable_workspace --cxxopt="-Wno-self-assign-overloaded" }
             cache_key: Bazel7bzlmod
             image: "us-docker.pkg.dev/protobuf-build/containers/common/linux/bazel:7.1.2-27cf7b86212020d7e552bc13b1e084abb971da75"
             targets: "//src/... //third_party/utf8_range/..."

CMakeLists.txt

@@ -186,7 +186,7 @@ set(protobuf_LINK_LIBATOMIC false)
 if (NOT MSVC)
   include(CheckCXXSourceCompiles)
   set(OLD_CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS})
-  set(CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS} -std=c++14)
+  set(CMAKE_REQUIRED_FLAGS ${CMAKE_REQUIRED_FLAGS} -std=c++17)
   check_cxx_source_compiles("
     #include <atomic>
     int main() {

cmake/README.md

@@ -12,16 +12,18 @@ the same actions can be performed using appropriate GUI tools.
 
 ## 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:
+By default, CMake will use whatever C++ version is the system default. Since
+protobuf requires C++17 or newer, sometimes you will need to explicitly override
+this. For example, the following:
 
 ```
-cmake . -DCMAKE_CXX_STANDARD=14
+cmake . -DCMAKE_CXX_STANDARD=17
 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).
+will build protobuf using C++17 (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
 
@@ -118,7 +120,7 @@ For example:
 ```console
 C:\Path\to\build\protobuf> cmake -S. -Bcmake-out \
                            -DCMAKE_INSTALL_PREFIX=/tmp/protobuf \
-                           -DCMAKE_CXX_STANDARD=14 \
+                           -DCMAKE_CXX_STANDARD=17 \
                            -DCMAKE_PREFIX_PATH=/tmp/absl  # Path to where I installed Abseil
 ```