Christopher Jefferson wrote:
A default hash function certainly can shuffle bits, to create a better distribution.
What does "better distribution" mean?
It might still be worth documenting this weakness however.
This is not a weakness. The definition of a hash function is that equal inputs produce the same output and different inputs should produce different outputs. This is exactly what this function does. If an algorithm doesn't work well with this hash function, this is a sign that the algorithm itself has a weakness, because it doesn't work well with functions that satisfy the requirements for a hash function. In other words, the algorithm places additional requirements on its hash function, requirements that a hash function author has no way of knowing. Now, to be honest, I know what the perceived defect is, that the default hash function is not cryptographic enough. In particular, that a single bit change in the input results in a predictable single bit change in the output, whereas one would like it to result in random bit changes all over the size_t. But, again, if your algorithm requires this property, it is not generic and will not necessarily work with a hash function written by someone who is not aware of this requirement. If you require the output bits to be properly shuffled, do that yourself, this is something that all hash functions may potentially need. Which is basically what you said. :-)
I would say not, as it is simple to add once a shuffle/distribution function, which can be applied to the hash, if the user wants one.