Qt 5.12.6, even though it's advertised as being able to be built with c++11, actually requires c++14 on macOS...
Apparently building with c++11 is considered a "corner case
"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."
but c++14 is available everywhere (even centOS). it seems the only reason bitcoin core didn't upgrade to c++14 is because people want to have less bumps and the next bump should therefore be c++17 as it is a feature upgrade and not a "nits issue"