foldr/nil rule not applied consistently

I just had a project where it made a difference whether I add

{-# RULES "foldr/nil" forall k n . GHC.Base.foldr k n [] = n #-}

to my file or not, despite this rule already being present in the library.

I tried to minimize the problem and came up with this:

foo1 (f, fs) (x, xs) = (f x, map ($x) fs ++ map f xs)
foo2 f fs x xs = (f x, map ($x) fs ++ map f xs)

test1 x xs = foo1 (id, []) (x, xs)
test2 x xs = foo2 id [] x xs
test3 x xs = (id x, map ($x) [] ++ map id xs)

test2 and test3 nicely optimize the map … [] ++ away, but test does not.

(In this minimized example, adding the rule again locally does *not* help, but there is still something fishy.)

Also, in all cases, map id remains, which should not be the case.

Trac metadata

Trac field	Value
Version	8.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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