Building 2027 with Apple Clang 21 fails

[thatcomputerguy0101]

Describe the bug
When trying to build the WPILib 2027 branch or v2027.0.0-alpha-4 on MacOS 26, clang 21.0.0, both the gradle and cmake builds fail due to the following error:

error: unknown warning option '-Wno-fixed-enum-extension'; did you mean '-Wno-bit-int-extension'? [-Werror,-Wunknown-warning-option]

I have not tried the Bazel build.

To Reproduce
Steps to reproduce the behavior:

1. ./gradlew build

Expected behavior
The build on an unmodified clone should be successful.

Desktop (please complete the following information):

• OS: MacOS 26
• Compiler: clang 21.0.0

[ThadHouse]

Does main fail too? That flag has been there for years, and should be there on main as well.

[ThadHouse]

Also, the cmake build should be working. We only apply that if clang < 19

[ThadHouse]

Also, are you sure you're running clang 21? Xcode is currently on AppleClang 17. We don't currently support non AppleClang versions on macOS.

[thatcomputerguy0101]

This is what I get from clang --version:

Apple clang version 21.0.0 (clang-2100.0.123.102)
Target: arm64-apple-darwin25.3.0
Thread model: posix
InstalledDir: /Applications/Xcode.app/Contents/Developer/Toolchains/XcodeDefault.xctoolchain/usr/bin

However, logging CMAKE_CXX_COMPILER_VERSION from cmake reports version 17.0.0.17000603.

Main fails with different, more varied errors. I attached the errors from a gradle wpimath build.

MacOS 26 wpimath 26 build errors.txt

[rzblue]

Can you fully delete the CMake build directory and reconfigure the CMake build? If the build directory was configured prior to updating macos it could be caching the versions and build commands

[thatcomputerguy0101]

Can you fully delete the CMake build directory and reconfigure the CMake build?

Thanks, that fixed it for 2027 + cmake. Gradle still fails with the same '-Wno-fixed-enum-extension' error though.

[PeterJohnson]

Can you confirm it's fixed now that #8752 is merged?

[thatcomputerguy0101]

Yep, that fixes the -Wno-fixed-enum-extension error. The version of ImGui that is bundled for Glass seems too old to build with Clang 21 though.

[thatcomputerguy0101]

Similarly, gtest fails to compile due to this:

.../allwpilib/thirdparty/googletest/include/gtest/gtest-printers.h:502:35: error: implicit conversion from 'char8_t' to 'char32_t' may change the meaning of the represented code unit [-Werror,-Wcharacter-conversion]
  502 |   PrintTo(ImplicitCast_<char32_t>(c), os);
      |           ~~~~~~~~~~~~~           ^

[PeterJohnson]

Working on fixes in #8756 (cmake only at the moment).

[thatcomputerguy0101]

I'm probably about 70% through updating ImGui and ImPlot. Still need to update the backend scripts, primarily removing CreateFontsTexture/DestroyFontsTexture.