cc1plus: warning: '-Werror=' argument '-Werror=implicit-int' is not valid for C++

[adfernandes]

I recently started testing the gcc-arm-none-eabi-9-2019-q4-major compiler in a C++ project, and came across the following for my C++ translation units:

cc1plus: warning: '-Werror=' argument '-Werror=implicit-int' is not valid for C++

This is a known change in gcc 9 and will likely is or will soon be a part of clang.

To Reproduce
Enable C++ and add a C++ file to the project.

Expected behavior
The -Werror=implicit-int option should not be enabled for C++ translation units.

Impact
This is an annoyance because Zephyr appears to really pride itself on "zero warning" builds, which is expected of possible mission- or safety-critical embedded systems.

Environment

• macOS and Linux
• gcc-arm-none-eabi-9-2019-q4-major
• Zephyr v2.1.0-rc3-6-gf570f19bef

First Steps Fixing

I'm volunteering to help fix this, if some kind person can give me some pointers!

The macro toolchain_cc_warning_error_implicit_int is defined

• here for gcc, and
• here for clang

The macro toolchain_cc_warning_error_implicit_int is enabled here

Now, following the chain:

• toolchain_cc_warning_error_implicit_int calls zephyr_cc_option
• zephyr_cc_option calls target_cc_option
• target_cc_option calls target_cc_option_fallback

Now the problem is (I think) that target_cc_option_fallback checks to see if the specified option is valid for C and CXX and possibly ASM. But the -Werror=implicit-int option only produces a warning, not an error in the test, so it's allowed through (for C++).

I believe that the GCC release notes say that this will soon become a build-breaking error, not a warning, so it really does need to be fixed sometime.

But for the life of me I can't decipher that kind of CMakeFoo without some help...

[vanwinkeljan]

@SebastianBoe could you provide some advice?

[kyberdin]

I'm also encountering this issue when compiling a project with C++ sources, but with an additional warning:

cc1plus: warning: command line option '-Wno-pointer-sign' is valid for C/ObjC but not for C++
cc1plus: warning: '-Werror=' argument '-Werror=implicit-int' is not valid for C++

Not surprisingly, if I comment out target_warnings.cmake:22 and target_warnings.cmake:94 for GCC then the warnings no longer get printed.

Am I overlooking something in the configuration for disabling these warnings?

OS: Ubuntu 18.04.5
Zephyr Version: 2.3.0
Compiler Version: arm-zephyr-eabi-g++ (crosstool-NG 1.24.0.37-3f461da-dirty) 9.2.0