Bug#1113237: llvm-toolchain-19: FTBFS with CMake 4

[Timo Röhling]

Hi,

* Matthias Klose <doko at debian.org> [2025-10-25 19:25]:
>I haven't seen any upstream support for cmake 4.x, not even in the 
>trunk (llvm-toolchain-snapshot). So if you upgrade to cmake 4.x, all 
>LLVM package stay unbuildable for some unseen time. What do you 
>suggest?

The actual bug is that LLVM {17,18,19} try to build shared libraries for 
WebAssembly, which does not support them. CMP0164 [1] turned this into a 
fatal error, and CMake 4 implicitly activates this policy in export 
files [2], which is why this became a FTBFS.

I'm going to ask CMake upstream for advice, but ultimately this is 
something that LLVM upstream needs to fix. Frankly, I would find it 
surprising if they have not addressed this already, and I would further 
assume that such a fix can be backported to the older releases.

If that is not possible for some reason, I might be convinced to 
introduce a cmake-3 legacy package as hotfix. This cannot be a permanent 
solution, though, and I'd rather avoid that maintenance burden 
altogether.


Cheers
Timo

[1] https://cmake.org/cmake/help/latest/policy/CMP0164.html
[2] https://gitlab.kitware.com/cmake/cmake/-/commit/10f8c6a2742500ddc708091f194d218a37543129


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