throwIO gets subsumed by a later imprecise exception

The code below exits with exception "Boom" when compiled with no options (the expected behavior, since throwIO should always subsume exceptions that come later in the IO monad), but with a head-of-empty-list exception when compiled with -O. Similar code has shown this problem on 7.2 and 7.4.

{-# LANGUAGE DeriveDataTypeable #-}
import Control.Exception (throwIO, Exception)
import Control.Monad (when)
import Data.Typeable (Typeable)

data Boom = Boom deriving (Show, Typeable)
instance Exception Boom

main = do
    args <- return []

    -- Should throw this exception.
    when (length args /= 1) (throwIO Boom)

    -- With -O, instead throws this one from head [].
    let n = read (head args)
    when (n < 0) (throwIO Boom)

    return (fromInteger n :: Int)

Trac metadata

Trac field	Value
Version	7.6.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
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