Implicit parameter constraints not inferred

To be fair, the ghc docs do not mention constraint inference for implicit parameters. OTOH they do not say that signatures must be given explicitly if I use implicit parameters. However, I always thought that this would be ok:

main = let ?b = True in use_b

--use_b :: (?g::Bool) => IO () use_b = print ?b

It isn't: ghc -fimplicit-params says Unbound implicit parameter (?b :: a) arising from use of implicit parameter `?b' at TestBug.hs:4 In the first argument of `print', namely `?b' In the definition of `use_b': use_b = print ?b It works if I uncomment the signature. I also tried

use_b = print (?b :: Bool)

but to no avail. Using ghc-6.2.2, btw. My question: Is this as it should be or is it a bug? Ben