I just came across this gemm on the Numpy mailing list:

'''

The cast indeed discards the imaginary part, and this may not be the
intended behavior in all cases, hence the warning. This warning can be
turned off in the standard way:

>>> import warnings
>>> warnings.simplefilter('ignore', np.ComplexWarning)

'''

I don't know what package provides the warnings module or how it works, but if numpy can use it, then Theano should use that rather than our own home-brew approach that we've started using.

James