2) Change the arguments to "void*" and "const void*", respectively; indicating that the arguments are not checked, since that is the caller's responsibility.

There are probably better options than voit * and const void *.

One method would be to duplicate the classes by deriving from the original but with a new name (and making no interface changes) which indicates the changes in responsibilities -- for example, a callee passed a Variant would be resonsible for validation checks, but a callee passed a CheckedVariant would not, safe in the knowlege that it was the callers responsibility.

If you want to get a bit fancy, you can structure things such that you can only get a CheckedVariant through a method on a nominal Variant, and enforce checking through that method (you might have different flavors of CheckedVariant as a result) if your checks are regular (as opposed to ad-hoc). You might further extend this through the CheckedVariant constructor or a conversion operator so that this can happen transparently at the callsite of methods taking CheckedVariants. You can further sprinkle in appropriate use of copy/move constructors and assignment operators to express the lifetime patterns that are appropriate.

That would reduce noise in the code and help spare your own human efficiency, but it also centralizes checking so that you can easily swap in asserts or what-have-you. When you detect that a conversion is not possible, all the advice above about failing immediately and loudly applies.