When declaring native callback functions with the JNI type system, then we actually register a helper function with variadic arguments, and implement that to forward the arguments to the declared function, which then might use higher-level types as arguments. We deduce those higher-level types through a variadic template, and use std::tuple as well as std::apply to generate the calls. Simplify the implementation by using std::make_tuple, and replace q20:remove_cvref_t with std::decay_t; this is what std::make_tuple uses, and we don't need to maintain functions and arrays as such. Found during 6.7 header review. Pick-to: 6.7 Task-number: QTBUG-119952 Change-Id: I7cd206c6b372c2ec62a10feb5f9253f5607f01a9 Reviewed-by: Marc Mutz <marc.mutz@qt.io>