Currently, XCBProtocolRequest has an isvoid flag. I expect more flags to become useful later -- the current motivating example is a 'multiple replies' flag. So isvoid should become a short or int bitmask. short is probably better so the structure can be packed smaller, and 16 flags probably will suffice.
Replacing xcb_protocol_request_t.isvoid with a uint8_t "flags" bitmask would be ABI-compatible, as long as isvoid is the least-significant bit, but it would need a bit of care to stay API-compatible. This may still be worth doing.
I don't think there's any point until we actually want a second flag, at which point yes, it's fine.
If we haven't needed one until now, we presumably never will.
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