After a bit of a false start last week I've set the master branch--the one that we'll release in a few months as GnuCash 4.0--to build its C++ files with C++17.
From a coding standpoint that pretty much affects only Geert and me: No one else is actively working with C++.
From a building and using standpoint it means that you're going to need a fairly recent distro or operating system release: The required compilers are gcc 8.0 and clang 6.0, and boost must in general be 1.67 or later. I say in general because we found that although Ubuntu 18.04 *nominally* ships with boost 1.65, they've patched the one library (boost::locale) that had a C++17 incompatibility until 1.67 so GnuCash builds. There may be others that have done something similar.
Other distros we tested on: Debian Buster, Bullseye, and Sid; Fedora 28, 29, and 30; OpenSuSE Tumbleweed (Leap 15.2 doesn't work). OpenBSD doesn't work (boost is too old).
libboost 1.65.1 (it will likely be patched as it is Ubuntu 18.04 based.
There is also a ppa where libboost 1.67 is available for 18.04
https://asciinema.org/a/199344 if you are not averse to installing from non