Having some static constraints around <expr> makes it a lot easier to analyze though. Both for humans and compilers. Otherwise you end up with situations like

Having some static constraints around <expr> makes it a lot easier to analyze though. Both for humans and compilers. Otherwise you end up with situations like C++ templates, that can expand to anything. With long confusing errors, unable to distinguish if it’s an error in the usage, the definition or the call-site.