theano: gradient where cost is imag(x)

问题

If I have a cost that is the imaginary part of a complex number, trying to obtain the gradient with theano I get the following error:

TypeError: Elemwise{imag,no_inplace}.grad illegally returned an integer-valued variable. (Input index 0, dtype complex128)

Is it not possible to use the imaginary part as cost despite it being a real-valued cost?

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Edit. Minimal working example

import theano.tensor as T
from theano import function
a = T.zscalar('a')
f = function([a], T.grad(T.imag(a),a))

I would expect this to work as T.imag(a) is a real scalar cost..

来源：https://stackoverflow.com/questions/31731711/theano-gradient-where-cost-is-imagx