Deriving Data does not work for attached code

I get the following error when I try to derive Data for the attached code:

Main.hs:17:66:
    Couldn't match expected type `Bar (D a b)'
                with actual type `t' a1 b1'
    Expected type: Maybe (c (Bar (D a b)))
      Actual type: Maybe (c (t' a1 b1))
    In the expression: gcast2 f
    In an equation for `dataCast2': dataCast2 f = gcast2 f

Trac metadata

Trac field	Value
Version	7.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
BlockedBy
Related
Blocking
CC	mmitar@gmail.com
Operating system
Architecture

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