I'm sorry for spamming code and not providing a distilled example, but honestly I have no idea how to even begin pinpointing this.

I'm writing a library and hosting it on Github: https://github.com/marcinmrotek/pipes-key-value-csv which compiles fine, but when I try to compile the test suite, GHC crashes with this error message:

/home/marcin/haskell/pipes-key-value-csv/.stack-work/dist/x86_64-linux/Cabal-1.22.4.0/build/Pipes/KeyValueCsv/Internal/KeyValue.hi
Declaration for $fFromKeyValuesf:3:
  Iface type variable out of scope:  k
Cannot continue after interface file error

when I uncomment the

. KV.foldHeader

line in https://github.com/marcinmrotek/pipes-key-value-csv/blob/master/test/Test/KeyValue.hs

The thing is, this function does not even use the "FromKeyValues" type class mentioned - https://github.com/marcinmrotek/pipes-key-value-csv/blob/master/src/Pipes/KeyValueCsv/Internal/KeyValue.hs

KV.parseLine, which does, compiles fine (the test suite runs and fails with "Prelude: undefined". They're both defined here: https://github.com/marcinmrotek/pipes-key-value-csv/blob/master/src/Pipes/KeyValueCsv/KeyValue.hs

I've tried "stack clean", reinstalling both GHC and stack, installing my package globally with cabal-install (although I had to fix the validation-0.5.1 package by removing the Safe pragma; I've tried to use stack again, pointing it to the exact same code for validation-0.5.1, to no avail), the result is always the same - the library compiles, the test suite doesn't.