STL algorithms, in general, don't specify how often the function
objects passed to them are copied during the run of the
algorithm.
While generate_n is above any reasonable suspicion of copying the
function object after the first invocation, passing a mutable lambda
containing the counter is still an anti-pattern we don't want people
to copy.
Fix in the usual way, by keeping the counter external to the lambda.
As a drive-by, replace post- with pre-increment.
Amends dc091e7443.
Pick-to: 6.5 6.2
Change-Id: I9c44e769fd41e5f7157179a2be4c3534424cf913
Reviewed-by: Ivan Solovev <ivan.solovev@qt.io>
edc953948c