In TcArrows, the following code

    tc_cmd w_tv (cmd, i, b, tup_ty, s)
      = do { tup_ty' <- zonkTcType tup_ty
	   ; let (corner_ty, arg_tys) = unscramble tup_ty'

		-- Check that it has the right shape:
		-- 	((w,s1) .. sn)
		-- where the si do not mention w
	   ; checkTc (corner_ty `eqType` mkTyVarTy w_tv && 
		      not (w_tv `elemVarSet` tyVarsOfTypes arg_tys))
		     (badFormFun i tup_ty')

	   ; tcCmdTop (env { cmd_arr = b }) cmd arg_tys s }

is wrong. In particular, the check elemVarSet w_tv (tyVarsOfTypes arg_tys) does not make sense in the presence of deferred constraints.

There is no testcase for this at the moment, but in the ghc-kinds branch both T5283 and arrowform1 fail because of this. The temporary fix was:

           ; _bogus <- unifyType corner_ty (mkTyVarTy w_tv)
	   ; checkTc (not (w_tv `elemVarSet` tyVarsOfTypes arg_tys))
		     (badFormFun i tup_ty')

This solves only T5283 and arrowform1; other problems might arise in the future, so this should be fixed properly.