Currently, functions can only carry one attribute of each category. Assuming there were an inverse for every attribute, the compiler wouldn't let you mark a function e.g. both @system and @safe.

What would that even mean?

• It would only be possible for function definitions, not prototypes.
• It enables the checks for the guarantee-making attribute, but be typed and mangled as lacking the guarantee-making attribute, e.g. a @system @safe function definition is checked for @safe, but is typed and mangled @system.

Why would you want this?

• Not often, sure, but for a virtual base class method, it makes sense. If it has a @safe implementation, having those checks enabled makes sense. Allowing derived classes to override it with a @system implementation also makes sense, but both together isn't possible.
• And if we're at it, if the base-class method is actually @safe (because it's provably checked for it), an overrider might just define a @safe override directly and be calling the base class method, non-virtually, rendering the call @safe. That would bind classes derived from that class to being @safe, but that's intended. And a non-trivial overrider may be @system @safe as well for the same reason.