platform.h error when building with GCC 13 using C++14

[jaskij]

Working or some minor improvements, and wanted to test them using C++14. Forced it in CMakeLists.txt. This started erroring out in platform.h because char8_t is a reserved identifier in C++20. The only way to get rid of the error (which seems a false positive given the codebase) was to add

add_compile_options(-Wno-c++20-compat)

CMake patch to force C++14:

--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -32,6 +32,8 @@ add_library(etl::etl ALIAS ${PROJECT_NAME})
 
 include(GNUInstallDirs)
 
+add_compile_options(-std=c++14)
+
 target_include_directories(${PROJECT_NAME} ${INCLUDE_SPECIFIER} INTERFACE
         $<BUILD_INTERFACE:${CMAKE_CURRENT_SOURCE_DIR}/include>
         $<INSTALL_INTERFACE:${CMAKE_INSTALL_INCLUDEDIR}>

Error:

[24/303] Building CXX object test/CMakeFiles/etl_tests.dir/murmurhash3.cpp.o
FAILED: test/CMakeFiles/etl_tests.dir/murmurhash3.cpp.o 
/usr/bin/c++ -DETL_DEBUG -I/home/jaskij/projects/deps/etl/test/../include -isystem /home/jaskij/projects/deps/etl/test/UnitTest++/.. -std=gnu++20 -std=c++14 -fno-omit-frame-pointer -fno-common -Wall -Wextra -Werror -Wfloat-equal -Wuseless-cast -Wshadow -Wnull-dereference -MD -MT test/CMakeFiles/etl_tests.dir/murmurhash3.cpp.o -MF test/CMakeFiles/etl_tests.dir/murmurhash3.cpp.o.d -o test/CMakeFiles/etl_tests.dir/murmurhash3.cpp.o -c /home/jaskij/projects/deps/etl/test/murmurhash3.cpp
In file included from /home/jaskij/projects/deps/etl/test/murmurhash3.cpp:10:
/home/jaskij/projects/deps/etl/test/../include/etl/platform.h:361:25: error: identifier ‘char8_t’ is a keyword in C++20 [-Werror=c++20-compat]
  361 |   typedef uint_least8_t char8_t;

[jaskij]

using ETL_CXX_STANDARD to set the C++ version causes the same error, with the same workaround

[jwellbelove]

Try defining ETL_NO_SMALL_CHAR_SUPPORT as 0 in etl_profile.h or in the compiler settings.

[jaskij]

Won't that create issues elsewhere? Since it's simply a forward-compatibility warning.

[jwellbelove]

It tells the ETL that the compiler has a native char8_t type.

[jaskij]

Ah! But it doesn't have char8_t.

I'm using GCC 13, but compiling in C++14 mode. This is basically a warning saying that the line would be an error if it was compiled in C++20 mode. Which won't happen because of the #ifdefs, but GCC seems to miss it.

[jwellbelove]

We may have to disable this warning, as I can't see any way to solve this other than not defining char8_t for <C++20, which is not a real solution.

[jwellbelove]

What is the GCC warning number for this?

[jaskij]

As far as I'm aware, GCC does not number their warnings, instead using names. I've opened #909 with a proposed solution.