Does not build with base 4.9

The base bounds for 0.5.6.0 are base (>=4.9 && <4.20), but it does not build with 4.9.

$ ghcup run --ghc 8.0.2 -- cabal repl -b 'array == 0.5.6.0'
...
Build profile: -w ghc-8.0.2 -O1
In order, the following will be built (use -v for more details):
 - array-0.5.6.0 (lib) (requires build)
 - fake-package-0 (lib) (first run)
Starting     array-0.5.6.0 (lib)
Building     array-0.5.6.0 (lib)

Failed to build array-0.5.6.0.
Build log (
...
):
Configuring library for array-0.5.6.0..
Preprocessing library for array-0.5.6.0..
Building library for array-0.5.6.0..
[ 1 of 15] Compiling Data.Array.Base  ( Data/Array/Base.hs, dist/build/Data/Array/Base.o )

Data/Array/Base.hs:53:1: error:
    Failed to load interface for ‘GHC.Ix’
    Perhaps you meant
      GHC.IO (from base-4.9.1.0)
      GHC.Int (from base-4.9.1.0)
      GHC.ST (from base-4.9.1.0)
    Use -v to see a list of the files searched for.
Error: cabal: Failed to build array-0.5.6.0 (which is required by
fake-package-0). See the build log above for details.

Discovered when I tried to check that !30 (merged) builds on 4.9.