Round-trip of `intptr_t` through `void *` isn't explicitly guaranteed by C99. There are implementation-defined behaviors for integer to `void *`, and vice versa. Only a round-trip of `void *` through `intptr_t` is guaranteed, not vice versa. We seem to depend on being able to store an integer in a `void *` from time to time, e.g., to store integers in a smartlist. We should run some automated tests to make sure this works as we expect, at least until we stop needing this behavior. (See also legacy/trac#23714.)