bug in default implementation of popCount

base 4.5.0.0 (http://hackage.haskell.org/packages/archive/base/4.5.0.0/doc/html/src/Data-Bits.html#popCount) gives the default implementation of popCount of the Bits class like this:

popCount          :: a -> Int
popCount = go 0
    where
        go !c 0 = c
        go c w = go (c+1) (w .&. w - 1)

Alas, .&. binds more tightly than -, thus the last expression parses as ((w .&. w) - 1) not as (w .&. (w-1)), as it was intended.

At the least, this causes popCount :: Integer -> Int to behave erratically:

Prelude Data.Bits> map popCount [2::Integer .. 4]
[2, 3, 4]

Trac metadata

Trac field	Value
Version	7.4.1
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
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