Replacing ICC C++14 with C++17 (#3570)

* Replacing ICC C++14 with C++17

* ICPC: -diag-disable:conversion

Try to suppress the `-Werror-all` promotion in `#include <variant>`

* Trying `-DPYBIND11_WERROR=OFF`

* Trying `-Wno-conversion` (todo: make specific to C++17 if it works)

* Inserting -Wno-conversion only for C++17
Co-authored-by: Axel Huebl <axel.huebl@plasma.ninja>

.github/workflows/ci.yml

@@ -559,37 +559,37 @@ jobs:
         set +e; source /opt/intel/oneapi/setvars.sh; set -e
         cmake --build build-11 --target test_cmake_build
 
-    - name: Configure C++14
+    - name: Configure C++17
       run: |
         set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake -S . -B build-14     \
+        cmake -S . -B build-17     \
         -DPYBIND11_WERROR=ON    \
         -DDOWNLOAD_CATCH=ON     \
         -DDOWNLOAD_EIGEN=OFF    \
-        -DCMAKE_CXX_STANDARD=14             \
+        -DCMAKE_CXX_STANDARD=17             \
         -DCMAKE_CXX_COMPILER=$(which icpc)  \
         -DPYTHON_EXECUTABLE=$(python3 -c "import sys; print(sys.executable)")
 
-    - name: Build C++14
+    - name: Build C++17
       run: |
         set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake --build build-14 -j 2 -v
+        cmake --build build-17 -j 2 -v
 
-    - name: Python tests C++14
+    - name: Python tests C++17
       run: |
         set +e; source /opt/intel/oneapi/setvars.sh; set -e
         sudo service apport stop
-        cmake --build build-14 --target check
+        cmake --build build-17 --target check
 
-    - name: C++ tests C++14
+    - name: C++ tests C++17
       run: |
         set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake --build build-14 --target cpptest
+        cmake --build build-17 --target cpptest
 
-    - name: Interface test C++14
+    - name: Interface test C++17
       run: |
         set +e; source /opt/intel/oneapi/setvars.sh; set -e
-        cmake --build build-14 --target test_cmake_build
+        cmake --build build-17 --target test_cmake_build
 
 
   # Testing on CentOS (manylinux uses a centos base, and this is an easy way

tests/CMakeLists.txt

@@ -327,6 +327,9 @@ function(pybind11_enable_warnings target_name)
     elseif(CMAKE_CXX_COMPILER_ID MATCHES "(GNU|Clang|IntelLLVM)")
       target_compile_options(${target_name} PRIVATE -Werror)
     elseif(CMAKE_CXX_COMPILER_ID STREQUAL "Intel")
+      if(CMAKE_CXX_STANDARD EQUAL 17) # See PR #3570
+        target_compile_options(${target_name} PRIVATE -Wno-conversion)
+      endif()
       target_compile_options(
         ${target_name}
         PRIVATE