Gentoo's Bugzilla –
Attachment 575586 Details for
Bug 685446app-misc/recoll-1.25.15 - In file included from common/rclconfig.cpp:44: /usr/lib/gcc/x86_64-pc-linux-gnu/9.1.0/include/g++-v9/bits/stl_tree.h:987:16: error: static assertion failed: comparison object must be invocable as const