Trait implemented, trait is not implemented,

I need to implement a trait PrimeField for a struct.
A convenient sample implementation is provided by the top-level crate description:

#[PrimeFieldModulus = "52435875175126190479447740508185965837690552500527637822603658699938581184513"]
#[PrimeFieldGenerator = "7"]
#[PrimeFieldReprEndianness = "little"]
struct Fp([u64; 4]);

Then we can construct a convenient newtype struct wrapping the Poseidon Hash function:

pub struct PosHash<'a>(pub Poseidon<'a, Fp>);

I see the sole error that ff::PrimeField is not implemented for Fp. Hmm. What if we sanity check, and write this instead?

pub struct PosHash<'a, F: ff::PrimeField>(pub Poseidon<'a, F>);

Same error. But how could that be? We just passed a generic parameter that implements PrimeField.

I'm pretty confused by this, what's going on here?

jk I've seen this before. I had to bump the version on the ff crate down a version, from 13 to 12.1 to be consistent with the Poseidon dependency on ff.

Closing this.