braces on single line if

rebase, remove maybe_unused tags

suppress clang-tidy warning about unused attrs (we use them in the next commit, temporarily removing them would cause multiple lines to be changed twice in two commitss)

rebase

fix var shadowing

suqashed with D18605

suqash with D18606 to avoid temporary variable shadowing (renaming flags -> m_flags fixes this)

fix some intermediate var shadowing

explicit

There is one more commit in core#27708 that cannot be backported yet, because of strong dependency on multiprocess/IPC backports. AFAICT it is a pure refactor with no behavior change, so not worth grabbing partially with workarounds that will make the multiprocess backports more confusing, better to get that one in the right order :
https://github.com/bitcoin/bitcoin/pull/27708/commits/61c569ab6069d04079a0831468eb713983919636

rebase, remove now useless cstdint in caches.h, remove unneeded other include (bad rebase conflict resolution)

rebase

keep the cstdint header for now, it will be used in D18583 and removed in D18584

rebase

rebase

rebase

use init-capture (see https://burnicki.pl/en/2021/04/19/capture-structured-bindings.html)

needs rebase after D18581 is fixed

looks like this includes c++20 code