Cap'n Proto 0.9 & 0.10 capnp/blob.h error: invalid operands to binary expression ('const char *' and 'kj::StringPtr')

[ryanofsky]

Cap'n Proto versions 0.9.x and 0.10.x are incompatible with clang 16 and later when -std=c++20 is used, causing error:

/usr/include/capnp/blob.h:176:74: error: invalid operands to binary expression ('const char *' and 'kj::StringPtr')

This was reported by willcl-ark in #194 (comment) and maflcko in #194 (comment)

This error can be avoided by any of the following:

• Upgrading to Cap'n Proto version 1.0 or newer which fixed this in capnproto/capnproto#1618, or applying the patch as done in shell.nix.
• Downgrading to Cap'n Proto version 0.8 or earlier, before capnproto/capnproto#1170 which triggers this issue starting in version 0.9.
• Downgrading to clang-15 or earlier, before P2468R2 which triggers this issue starting in version 16.
• Maybe using GCC (not sure what the status of this is with GCC)

Background

• Libmultiprocess only uses basic Cap'n Proto features, and for the most part doesn't rely on its c++20 behavior, though it did recently work around a c++20 GCC bug https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53499 in mpgen: Work around c++20 / capnproto 0.8 incompatibility #194
• Cap'n Proto started to improve it's C++20 support in capnproto/capnproto#1170 and capnproto/capnproto#1235 which were both included in version 0.9.0. But unfortunately these changes stopped working well when clang-16 was released because it implemented P2468R2.
• P2468R2 caused the compiler to stop generating reversed operator==(U,T) comparisons from operator==(T,U) if an operator!=(T,U) comparison was declared.
• This issue was fixed in capnproto/capnproto#1618, and the fix was expanded in capnproto/capnproto#1733. Both of these changes are included in Cap'n Proto versions 1.0 and later.

[maflcko]

Downgrading to clang-15 or earlier, before P2468R2 which triggers this issue starting in version 16.

That doesn't work with Bitcoin Core, which requires clang-16.

Maybe -DENABLE_IPC=OFF can be set temporarily for the 30.x release branch, only enabling it in depends (and thus the release binaries) by default for now? Though, I am not sure how many people are using Ubuntu Jammy (with system capnproto 0.8) or Debian Bookworm with clang.

[ryanofsky]

We could change ENABLE_IPC default, and there's been a lot of discussion about what this default should be in bitcoin/bitcoin#31802, bitcoin/bitcoin#33190, and bitcoin/bitcoin#33235.

The thing I'd like most like to do is make sure build-unix.md instructions work out of the box by default, and update them to mention incompatibilities if certain package versions won't work together. It looks like the Ubuntu Jammy and Debian Bookworm incompatibilities should both be mentioned, though I'm not clear whether the debian one is a clang-only issue or if it happens with gcc

[ryanofsky]

Another thing that could be nice to do in both cases is improve the build error messages and have them link to relevant issues and suggest using -DENABLE_IPC=OFF as a workaround if IPC features are not needed.