Export using user defined variables results in NULL

From http://stackoverflow.com/questions/41428627/phpmyadmin-export-with-generic-field

I have a query which works fine in view:

SET @c := 0;
SELECT ( SELECT @c := @c + 1 ) AS rowNum, email FROM HS_mailList;

However when I use the export function of the query result, the "rowNum" field shows NULL in the CSV. I guess it is NULL because the SET query is not processed and my "c" var is null. I think that's it, because phpMyAdmin shows the SELECT query only, after executing the SET and SELECT combination on top. On the other hand when clicking on "show sql-querybox" it still remembers both.

Using master, I tried the same on sakila with:
SET @c := 0;
SELECT ( SELECT @c := @c + 1 ) AS rowNum, first_name FROM actor;
I got the same incorrect result.

No source file or test is named in the report. Reproduce the issue with the provided SET and SELECT statements against sakila, compare the query view with the export path, and trace how the combined query is handled. Done means the CSV export preserves the computed rowNum values instead of emitting NULL.