I want to compose my ConversionTarget setup from calls to multiple populateSmthLegality(ConversionTarget &target). It work fine for trivial addLegalOp/addIllegalOp but for any non-trivial logic where markUnknownOpDynamicallyLegal is used this approach won’t work, as ConversionTarget can have only one (latest) markUnknownOpDynamicallyLegal callback.
My proposal is to change markUnknownOpDynamicallyLegal callback signature from bool(Operation*) to Optional<bool>(Operation*). Each markUnknownOpDynamicallyLegal will add callback to the list and when checking for legality this list will be traversed in reverse order until some callback return non-empty optional.