Strange "redundant" import

While working on !1487 (closed) I encountered a very strange issue.

Specifically, Haddock failed with:

compiler/iface/ToIface.hs-boot:6:1: error: [-Wunused-imports, -Werror=unused-imports]
    The import of ‘Var’ is redundant
      except perhaps to import instances from ‘Var’
    To import instances alone, use: import Var()
  |
6 | import Var ( TyCoVarBinder )
  | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^

While ToIface.hs-boot does contain this import, it most certainly is not redundant (as evidenced by the fact that GHC itself didn't complain, only haddock):

module ToIface where

import {-# SOURCE #-} TyCoRep
import {-# SOURCE #-} IfaceType( IfaceType, IfaceTyCon, IfaceForAllBndr
                               , IfaceCoercion, IfaceTyLit, IfaceAppArgs )
import Var ( TyCoVarBinder )
import VarEnv ( TidyEnv )
import TyCon ( TyCon )
import VarSet( VarSet )

-- For TyCoRep
toIfaceTypeX :: VarSet -> Type -> IfaceType
toIfaceTyLit :: TyLit -> IfaceTyLit
toIfaceForAllBndr :: TyCoVarBinder -> IfaceForAllBndr
toIfaceTyCon :: TyCon -> IfaceTyCon
toIfaceTcArgs :: TyCon -> [Type] -> IfaceAppArgs
toIfaceCoercionX :: VarSet -> Coercion -> IfaceCoercion
tidyToIfaceTcArgs :: TidyEnv -> TyCon -> [Type] -> IfaceAppArgs

As expected, the module fails to compile if the import of Var is removed. However, strangely the Haddock issue vanishes if the TyCoRep import is given an import list:

import {-# SOURCE #-} TyCoRep ( Type, TyLit, Coercion )

This is quite unexpected as TyCoRep's hs-boot file doesn't even import Var, which defines TyCoVarBinder.