I'm attaching 2 modules, both should be compiled with -O2. Let's look at the inteface of Foo:

  foo :: (GHC.Types.Int, GHC.Types.Int)
         -> [GHC.Types.Int]
         -> [GHC.Types.Int]
    {- Arity: 2, Strictness: U(U(L)U(L))L, Inline: INLINE (sat-args=2),
       Unfolding: InlineRule (2, False, False)
                  (\ ds :: (GHC.Types.Int, GHC.Types.Int) xs :: [GHC.Types.Int] ->
                   case @ [GHC.Types.Int] ds of wild { (i, n) ->
                   let {
                     step :: forall t.
                             ([t], GHC.Types.Int, GHC.Types.Int)
                             -> Data.Maybe.Maybe (([t], GHC.Types.Int, GHC.Types.Int), t)
                       {- Inline: INLINE[0] (sat-args=1) -}
                     = ...

Note how step, which is local to foo, has an INLINE pragma but no unfolding. Now, let's look at Bar. For some reason, GHC only inlines foo into bar in phase 1. This is probably unrelated but still surprising. The main problem, though, is that step, not having an unfolding, doesn't get inlined at all despite the INLINE pragma.