Warn about / refuse unsupported clang version #30947

On an older Intel macOS 13.7 machine:

cmake -B build
-- The CXX compiler identification is AppleClang 14.0.3.14030022

...

cmake --build build -j7

...

In file included from /Users/sjors/dev/bitcoin/src/crypto/muhash.h:9:
/Users/sjors/dev/bitcoin/src/uint256.h:133:19: error: call to consteval function 'util::ConstevalHexDigit' is not a constant expression
        auto lo = util::ConstevalHexDigit(*(str_it++));
                  ^

(etc)

Our minimum required clang is 16 so this is no surprise.

It would be good if cmake -B build checks the minimum version and at least throws a warning.

The documentation says "For macOS 11 (Big Sur) and 12 (Monterey) you need to install a more recent version of llvm". That should include macOS 13 and 14 now as well, see https://trac.macports.org/wiki/XcodeVersionInfo#macOS14

But I think as long as cmake warns about llvm being too old, anyone reading build-osx.md will probably figure out that they need to install a newer version via llvm. But it's a trivial doc change, see #30949.

There is no need to use brew to install clang/llvm. You can just pick the newer XCode: sudo xcode-select --switch /Applications/Xcode_15.0.app

You may have to upgrade to macOS 13.5 (or later), according to the release notes:

https://developer.apple.com/documentation/xcode-release-notes/xcode-15-release-notes

It would be good if cmake -B build checks the minimum version

We don't check compiler versions, we check for C++ compatibility (i.e CMAKE_CXX_STANDARD, CMAKE_CXX_STANDARD_REQUIRED, CMAKE_CXX_EXTENSIONS). If CMakes CMAKE_CXX_STANDARD check is incorrectly passing, when a compiler doesn't actually support C++20, we should file a bug upstream, so they can expand the checks they perform.

we should file a bug upstream, so they can expand the checks they perform

I used cmake version 3.30.3 (via Homebrew).

Even this doesn't trigger an error:

add_library(core_interface INTERFACE)
target_compile_features(core_interface INTERFACE cxx_std_20)

From googling around I get the impression this check isn't reliable, and you have to check for specific features. E.g. https://stackoverflow.com/questions/75296149/cmake-set-project-to-c-20#comment132865328_75296149

That sounds potentially more tedious than just waiting for compile to fail.

There is no need to use brew to install clang/llvm. You can just pick the newer XCode: sudo xcode-select --switch /Applications/Xcode_15.0.app

That only works if you install Xcode, otherwise you're stuck on whichever version of the command-line tools came with the OS.

You can't install Xcode via the App Store (on macOS 13). You have to download it via the Developer Center. Account required (the free one). You'll need version 15.2, because 15.4 required macOS 14.

So that's more tedious than using Homebrew, but arguably it's a cleaner approach.

There is no need to use brew to install clang/llvm. You can just pick the newer XCode: sudo xcode-select --switch /Applications/Xcode_15.0.app

That only works if you install Xcode, otherwise you're stuck on whichever version of the command-line tools came with the OS.

You can't install Xcode via the App Store (on macOS 13). You have to download it via the Developer Center. Account required (the free one). You'll need version 15.2, because 15.4 required macOS 14.

So that's more tedious than using Homebrew, but arguably it's a cleaner approach.

Well, I've never used Apple, so I don't know which is more tedious. But implying only the brew approach works would be wrong. Also, neither 15.2 nor 15.4 is "required". 15.0 works fine (and should work fine on macOS 13.5 as well).

From googling around I get the impression this check isn't reliable, and you have to check for specific features. E.g. https://stackoverflow.com/questions/75296149/cmake-set-project-to-c-20#comment132865328_75296149

That is what CMake does behind the scenes, i.e the tests in https://github.com/Kitware/CMake/tree/master/Tests/CompileFeatures. So if they are missing coverage for a certain compiler/cxx std combo, it's a bug they should address, not something we should have to test ourselves.

@fanquake do you mean this file? https://github.com/Kitware/CMake/blob/master/Tests/CompileFeatures/cxx_std_20.cpp

I'd start by looking at https://github.com/Kitware/CMake/blob/master/Tests/CompileFeatures/CMakeLists.txt, and the various combinations of compiler (versions) + feature tests.

Individual compile features are not provided for C++ 17 or later. See https://cmake.org/cmake/help/latest/prop_gbl/CMAKE_CXX_KNOWN_FEATURES.html#low-level-individual-compile-features

And I don't think they are tested either, because code should be using the feature test macros introduced in recent versions of C++.

It seems like they add cpp feature checks for various compilers if it's otherwise broken/doesn't always report the right version, i.e https://github.com/Kitware/CMake/blob/master/Modules/CMakeCXXCompilerId.cpp.in#L46, maybe they need to do the same for Apple Clang.

I'm not able to run fuzzing tests also because of this:

$ cmake --build build_fuzz
....
/bitcoin/src/uint256.h:133:19: error: call to consteval function 'util::ConstevalHexDigit' is not a constant expression
        auto lo = util::ConstevalHexDigit(*(str_it++));

Which clang version are we suposed to use?

$ clang --version
Ubuntu clang version 14.0.0-1ubuntu1.1

Which clang version are we suposed to use?

https://github.com/bitcoin/bitcoin/blob/master/doc/dependencies.md