`elemIndex` and `findIndex` still can't fuse

Ticket #14387 (closed) introduced a change to the implementation of listToMaybe to allow:

findIndex p = listToMaybe . findIndices p

to fuse.

However, to take advantage of this, it looks like we also need findIndex (and elemIndex) to be marked inlinable (or some similar step).

As a concrete example, the module:

module Foo where

import Data.List (findIndex)

foo :: Maybe Int
foo = findIndex (==999999) [1..1000000]

compiled with GHC 8.4.3 using -O2 produces the following unfused core:

foo_go
  = \ ds1_a2ws eta_a2wt ->
      case ds1_a2ws of {
        [] -> Nothing;
        : y_a2wx ys_a2wy ->
          case eqInteger# y_a2wx ds_r2we of {
            __DEFAULT -> foo_go ys_a2wy (+# eta_a2wt 1#);
            1# -> Just (I# eta_a2wt)
          }
      }

foo = foo_go (enumDeltaToInteger1 foo2 foo1) 0#

but if the definition of findIndex from Data.OldList is copied into the module or imported from another module with an INLINABLE pragma, then it fuses fine:

foo_go
  = \ x_a2Du eta_B1 ->
      case gtInteger# x_a2Du lim_r2Ey of {
        __DEFAULT ->
          case eqInteger# x_a2Du ds_r2Cv of {
            __DEFAULT -> foo_go (plusInteger x_a2Du foo1) (+# eta_B1 1#);
            1# -> Just (I# eta_B1)
          };
        1# -> Nothing
      }

foo = foo_go foo1 0#

Trac metadata

Trac field	Value
Version	8.4.3
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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