(See also the mailing list thread)

Static objects with no SRTs and pointer fields are currently ignored by evacuate(). This is efficient but causes trouble when checking alive-ness of static objects in isAlive(), which is used for (among other things) checking whether a weak's key has died. This causes leaks as value of a weak with static key will always be kept alive.

Here's a demonstration:

module Main where

import System.Mem.Weak (mkWeak, deRefWeak)
import System.Mem (performMajorGC)

mkKey :: IO String
mkKey = readFile "/dev/random"

-- mkKey :: IO Int
-- mkKey = return 1

main :: IO ()
main = do
    w <- mkKey >>= \k -> mkWeak k () Nothing
    performMajorGC
    performMajorGC
    performMajorGC
    deRefWeak w >>= print

The idea is that the first mkKey function returns a non-static string and this program prints Nothing. If I enable the second mkKey function the key is now a static object and the program prints Just ().