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  • Simon Peyton Jones's avatar
    Fix egregious error in eta-reduction of data families · 1160dc51
    Simon Peyton Jones authored
    This terrible and long-standing bug was shown up by Trac #11148.
    We are trying to eta-reduce a data family instance, so that we
    can then derive Functor or Generic.  But we were assuming, for
    absolutely not reason whatsoever, that the type variables were
    lined up in a convenient order.  The fact that it ever worked
    was a fluke.
    
    This patch fixes it properly.  Main change is in eta_reduce
    in TcInstDcls.tcDataFamInstDecl
    1160dc51