From ML thread: need @flatten to get decent performance, and @noinline to have compile times measured in minutes and not hours (literally, as @flatten can otherwise result in practically infinite recursive inlining).
https://github.com/D-Programming-GDC/GDC/commit/3e16d4d7673e4b3b8750df8bc92ce853199a2e78 https://github.com/D-Programming-GDC/GDC/commit/cfb656dc90649a2498c88eac7a30dad49ff32153