Christian Maeder wrote:

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There might be an issue with pre-processing, but that's beyond my cabal knowledge.

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Using cpphs version 1.1

After removing cpphs it worked!

cpphs-1.1 is extremely old (more than 3 years). The most recent release is 1.5, and it has many small bugfixes over 1.1, including some to do with preprocessing .hsc files. Regards, Malcolm