TLDR: Mention that the codebase is now compatible with C++17, and that the intention is to require C++17 starting with 0.22.0.
Following some discussion with Cory/Carl, and in #16684, I think this is the next step in the C++17 migration.
While #16684 mentions a gitian/Guix release with C++17, it’s not yet clear how that would be done. Are we just going to pass --enable-c++17 in gitian/Guix?. Are we changing our default in configure.ac?
According to the last comment in #16684, we wouldn’t be changing anything in depends:
No, everything (including depends) will stay at C++11.
However I don’t think we want to be mixing C++11 built dependencies, with a C++17 built bitcoind, if there is any potential for compatibility issues.
Instead, I’d suggest we build the 0.21.0 release as C++11, and do a complete switch to C++17 for 0.22.0. Also, if we actually wanted to use C++17 in depends for 0.21.0, we couldn’t without breaking C++11 compat (Qt). See below.
Here is a potential timeline/TODOs for the migration:
Potential Timeline
- 17 / 6 / 2020 - Today
- Some time prior to split-off:
- Confirm that compiling with C++17 works.
- Confirm that C++11 compatibility has not been broken.
- 1 / 11 / 2020
- 2 / 11 / 2020
- Merge an “incompatible with C++11” change into master.
- Switch configure to use C++17 mode by default.
- Update minimum compiler requirements. At least:
- Clang 5: https://clang.llvm.org/cxx_status.html#cxx17
- GCC 7: https://gcc.gnu.org/projects/cxx-status.html#cxx17
- While GCC has some support from 5, it seems a more complete support landed in GCC 7.
- https://gcc.gnu.org/gcc-7/changes.html#cxx
- Switch depends packages to use C++17 where applicable.
- Bump Qt from 5.9.x (no c++17 mode) to, likely, 5.15.x (LTS).
- Drop support for macOS < 10.14.x
- The c++ dylib shipped with macOS doesn’t support c++17, prior to macOS 10.14.
- Building Qt 5.12 or 5.15 in C++17 mode will also require a minimum macOS deployment target of 10.14. https://codereview.qt-project.org/c/qt/qtbase/+/283832.
- Begin merging PRs like #19183 and #19245.
- I’ve left some comments in #19183 if the macOS runtime issue interests anyone.
- 3 / 12 / 2020
- 0.21.0 released.
- Built as C++11.
- Contains warning in release notes that compiling 0.22.0 will require C++17.
- 3 / 6 / 2021
- 0.22.0 released.
- Full of C++17 code.
One thing worth noting, is that we cannot bump our Qt to a newer LTS for 0.21.0, without breaking C++11 compatibility. Qt 5.12 is not compilable in C++11 mode, as the project has started using C++14 features throughout at least the macOS portions of it’s codebase, and seemingly “forgotten” that the release is meant to be C++11 compatible. Upstream bug here: https://bugreports.qt.io/browse/QTBUG-77310.
Building Qt requires C+11, at a minimum, but in practice we use later features, usually under a feature define, or with a fallback of some kind. On platforms that support > C11, we’ve (apparently) not considered the fallback necessary, under the assumption C+14 is always available.
