allow duplicate deriving / standalone deriving
I would like to avoid conditional compilation. Currently I need to write:
#if MIN_VERSION_Cabal(1,18,0) -- cabal 1.18 has these instances #else deriving instance Typeable Flag #endif
Would it be sane to have the deriving instance do nothing if the imported instance that clashes was also derived?
reifyInstances together with a fixed #8100 (closed) could allow faking this feature.