Missing "Defined but not used" for recursive expressions

[guest]

a :: Int
a =
  let b = "foo"++b
  in  42

In this example b is not really used, but there is no warning by GHC. I assume this is so because b calls itself. If I remove ++b then I get the warning as expected.

(This reminds on the difference between a purely reference counting garbage collector and a correct one.)

[igloo]

Thanks for the report. Happily, 6.10.1 (and the HEAD) already warns about this correctly:

b.hs:4:6: Warning: Defined but not used: `b'