Strange associated types behaviour

strange_assoc_types.rs

use std::num::Float;

trait Foo {
    type A: Float;
    
    // Why is it that this doesn't work...
    fn sub_a(&self, a: <Self as Foo>::A, b: <Self as Foo>::A) -> <Self as Foo>::A {
        a - b
    }
    
    // ... and this does.
    fn sub_b(&self, a: <Self as Foo>::A, b: <Self as Foo>::A) -> <Self as Foo>::A {
        fn sub<T: Float>(a: T, b: T) -> T { a - b }
        sub(a, b)
    }
}

fn main() {}