Support enumerate
in conditional generators/comprehensions
#1525
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Trying to differentiate over a comprehension or generator over
enumerate
with a conditional errors because the tangent input will containnothing
instead ofTuple{Nothing, T}
, and the mappedlast
to extract theT
value fails.This PR adds a
back
method for this case which mapsnothing
tonothing
, and otherwise takinglast
.PR Checklist