Patch is specific of 5.7.
The root cause is mis-management of Item_field::result_field vs
Item_field::field; in 5.7, save_in_field_inner() uses result_field as
source; in 8.0 it uses field.
result_field has wrong data, leading to server exit; even if fixing
server exit is easy (by back-porting the fix for Bug#23183982),
then query's result is wrong, and fixing this would
require a back-port of the big code refactoring done in 8.0 (for Bug#26188578),
which isn't possible in 5.7-GA (too high risk).
Solution for 5.7: return error with suggestion of workaround.
In 8.0, everything works already.
No testcase as it's big. I verified the fix on my machine manually, though.