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Merge #19305: doc: add C++17 release note for 0.21.0

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f1d21ef1c3 doc: add C++17 release note for 0.21.0 (fanquake)

Pull request description:

  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](https://github.com/bitcoin/bitcoin/issues/16684#issuecomment-643778757) 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
      * [0.21.0 split off happens](https://github.com/bitcoin/bitcoin/issues/18947).
  * 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](https://github.com/bitcoin/bitcoin/issues/16684#issuecomment-643722538).
          * 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.

ACKs for top commit:
  MarcoFalke:
    ACK f1d21ef1c3 can't hurt to give an advance warning
  Sjors:
    ACK f1d21ef1c3
  laanwj:
    ACK f1d21ef1c3
  theStack:
    ACK f1d21ef1c3

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Wladimir J. van der Laan 2020-06-22 19:12:30 +02:00
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doc/release-notes.md
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@ -45,6 +45,11 @@ wallet versions of Bitcoin Core are generally supported.
Compatibility Compatibility
============== ==============
During this release cycle, work has been done to ensure that the codebase is fully
compatible with C++17. The intention is to begin using C++17 features starting
with the 0.22.0 release. This means that a compiler that supports C++17 will be
required to compile 0.22.0.
Bitcoin Core is supported and extensively tested on operating systems Bitcoin Core is supported and extensively tested on operating systems
using the Linux kernel, macOS 10.12+, and Windows 7 and newer. Bitcoin using the Linux kernel, macOS 10.12+, and Windows 7 and newer. Bitcoin
Core should also work on most other Unix-like systems but is not as Core should also work on most other Unix-like systems but is not as