This reverts commit b9cce12e76, which
broke
function_ref<void(int)> f = [](int i) { return i; };
ie. swallowing of return types.
We could maybe implement the same without invoke_r, with the same
manual if-constexpr that invoke_r has, but it would be a pointless
duplication across the two thunks we have, so just use invoke_r.
Add tests.
Pick-to: 6.4
Task-number: QTBUG-103739
Change-Id: I6034f05d813c06a25e8058ded5b6b62f3ca858b4
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
fa4d18b86c