Forall'd variable ‘$rcobox’ is not bound in RULE lhs

I tried to add this rule to the code of Data.Map:

{-# RULES
"mapKeysMonotonic/coerce" mapKeysMonotonic coerce = coerce
 #-}

but it would not go through:

libraries/containers/Data/Map/Base.hs:1800:1: warning:
    Forall'd variable ‘$rcobox’ is not bound in RULE lhs
      Orig bndrs: [k2, a, $rcobox]
      Orig lhs: let {
                  cobox_ab8H :: Coercible k2 k2
                  [LclId, Str=DmdType]
                  cobox_ab8H =
                    MkCoercible @ * @ k2 @ k2 @~ (<k2>_R :: k2 ~R# k2) } in
                mapKeysMonotonic @ k2 @ k2 @ a (coerce @ k2 @ k2 cobox_ab8H)
      optimised lhs: mapKeysMonotonic
                       @ k2
                       @ k2
                       @ a
                       (\ (tpl_B2 :: k2) ->
                          case cobox_ab8H of _ [Occ=Dead] { MkCoercible tpl_B3 -> tpl_B2 })

The same syntax works for mapping the second parameter, i.e.

{-# RULES
"map/coerce" map coerce = coerce
 #-}

(this still Data.Map’s map).

This blocks https://github.com/haskell/containers/pull/163

I’ll see if I can produce a stand-alone testcase.

Trac metadata

Trac field	Value
Version	7.11
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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