There are many instances in our code base where we could use the C++17 construct if constexpr.

Starting with PR match_system is a constexpr function. Thus many checks against match_system can be made if constexpr. In release mode, this is unlikely to help performance since the compiler optimizer can already know that it is dealing with a compile-time constant. However, it may help with code clarity and maybe have other benefits.

cc @anonrig