UArray Bool can perform "out-of-bounds" reads

Summary

Compile the following program with ghc -fcheck-prim-bounds -O Main.hs.

module Main (main) where

import Data.Array.Unboxed

arr :: UArray Int Bool
arr = listArray (2,7) (map even [2..7])

main :: IO ()
main = print (arr ! 5)

When run, it crashes with an error message like this one:

Main: internal error: Encountered out of bounds array access.
    (GHC version 9.4.4 for x86_64_unknown_linux)
    Please report this as a GHC bug:  https://www.haskell.org/ghc/reportabug
Aborted

Expected behavior

This is an in-bounds UArray access; the array library should correctly lower it to an in-bounds ByteArray# access.

---

Diagnosis: The basic problem is that the UArray i Bool stuff rounds the size of the underlying buffer to the nearest byte but actually does reads and writes at machine-word width.