I first would question what a user wants to do with a hashmap that uses polymorphic key-values of unknowable type at compile-time.

As a thought experiment, you could certainly have users define their own hash and equality functions and attach them to the table-entries themselves. On first thought, that sounds like it would be rife with memory safety issues.

At the end of the day, it is all just bytes. You could simply say that you will only key based on raw memory sequences.