[ferdymercury]

This Pull request:

Changes or fixes:

Alternative to #23160 that does not require any upstream or downstream patches to Clang.

This is a temporary solution, since building with LLVM.so is broken at the moment. (#20903 (comment)). It can be removed later if at some point that issue gets fixed.

In ROOT, for builtin_llvm=OFF we have the sequence

find_package(LLVM) # searchs for system package, which sets DYLIB to ON
set(LLVM_LINK_LLVM_DYLIB OFF) # override DYLIB to OFF, we want static
add_subdirectory(clang) # this will call find_package LLVM again

See #13420

Since LLVM22 though, this fails:
Even if we call for a second time set(LLVM_LINK_LLVM_DYLIB OFF) after the add_subdirectory line, it's not enough, because that variable was ON all throughout the child clang/CMakeLists.txt and has already configured some stuff with the wrong flag.

Avoid the issue by preventing the double-search of LLVM and thus the double overwrite, by temporarily overwriting the find_package(LLVM) calls.

Reproducer

docker run -it registry.cern.ch/root-ci/ubuntu2604:buildready
apt update
apt install llvm-22-dev libedit-dev libpolly-22-dev
git clone https://github.com/root-project/root root_src && cd root_src
git checkout master
mkdir build && cd build
cmake -Dbuiltin_llvm=OFF -Dminimal=ON ..
grep -r LLVM.so *

Checklist:

• tested changes locally
• updated the docs (if necessary)

Fixes #18387
Fixes #23155

[github-actions]

Test Results

    22 files      22 suites   3d 11h 49m 21s ⏱️
 3 879 tests  3 879 ✅ 0 💤 0 ❌
76 327 runs  76 327 ✅ 0 💤 0 ❌

Results for commit 80875d8.

♻️ This comment has been updated with latest results.

[guitargeek]

Thanks a lot for this new solution attempt, looks great!

• Fixes the builtin_llvm=OFF configuration without an additional LLVM/Clang patch, handling the problem from the ROOT side. @hahnjo is explicitly fine with that (#18387 (comment))
• Overriding find_package is a common practice, but it might cause problems for vcpkg o n Windows (see #8633). But since we're restricting this override to the builtin_llvm=OFF configuration that is typically not used on Windows, this is fine for a temporary workaround that we'll carry until we can link against shared LLVM

I just have a few minor change requests that should be addressed before I can approve this PR.

[guitargeek]

LGTM! See my previous comment for thorough review (#23371 (review))