Rule "fromIntegral/Int->Natural" is inconsistent

As noticed by @trac-vdukhovni in #20062 (comment 362750):

import GHC.Natural

i :: Int
i = -10

main :: IO ()
main = let n :: Natural
           n = fromIntegral i
        in print n

• with -O2: prints 18446744073709551606
• with -O0: throw "arithmetic underflow"

While it should always throw an exception (I guess? "fromIntegral" isn't precisely defined in the report).

GHC versions: 9.0, 9.2, HEAD