Building LLVM and MLIR with LLVM_REQUIRED_CXX_STANDARD c++23

[kykrueger]

Building llvm with c++23 seems to not yet be supported.
I'm keen on building it anyways to use 'import std;' in NebulaStream which uses LLVM with MLIR as a dependency.

I'm making patches to get the build working based on the llvmorg-20.1.8 release, and many of them seem to still be relevant when building against the release candidates for 21 and 22.

When switching the LLVM_REQUIRED_CXX_STANDARD variable to 23 on the 20.1.8 release many errors are encountered.
Most of the issues relate to the constexpr changes of unique_ptr in c++23.

I'd like to make a PR to share the changes I am making to get this working and see if any of them could be useful for the next 20.x.x release or other major releases.

[llvmbot]

@llvm/issue-subscribers-mlir

Author: Kyle Krüger (kykrueger)

Building llvm with c++23 seems to not yet be supported. I'm keen on building it anyways to use 'import std;' in [NebulaStream](https://github.com/nebulastream/nebulastream) which uses LLVM with MLIR as a dependency.

I'm making patches to get the build working based on the llvmorg-20.1.8 release, and many of them seem to still be relevant when building against the release candidates for 21 and 22.

When switching the LLVM_REQUIRED_CXX_STANDARD variable to 23 on the 20.1.8 release many errors are encountered.
Most of the issues relate to the constexpr changes of unique_ptr in c++23.

I'd like to make a PR to share the changes I am making to get this working and see if any of them could be useful for the next 20.x.x release or other major releases.

[EugeneZelenko]

Only most recent release (21 as of now) is maintained.

[kykrueger]

Thanks for the info, I'll rebase onto 21.

[nikic]

All patches must go into the main branch first anyway.