Returning a different class in #[new] method

[benruijl]

In Python you can do:

class A:
    pass
class B:
    def __new__(self, name):
        return A()

and I would like to do the same with pyo3. The following attempt does not work however:

#[pyclass]
pub struct A {}

#[pyclass]
pub struct B {}

#[pymethods]
impl B {
    #[new]
    pub fn __new__() -> A {
        A {}
    }
}

as I get:

error[E0277]: the trait bound `PyClassInitializer<B>: From<A>` is not satisfied
   --> src/api/python.rs:288:1
    |
288 | #[pymethods]
    | ^^^^^^^^^^^^ the trait `From<A>` is not implemented for `PyClassInitializer<B>`
    |
    = help: the following implementations were found:
              <PyClassInitializer<S> as From<(S, B)>>
              <PyClassInitializer<T> as From<T>>
    = note: required because of the requirements on the impl of `Into<PyClassInitializer<B>>` for `A`
    = note: required because of the requirements on the impl of `IntoPyCallbackOutput<PyClassInitializer<B>>` for `A`
    = note: this error originates in the attribute macro `pymethods` (in Nightly builds, run with -Z macro-backtrace for more info)

Does anyone know how to implement this?

[davidhewitt]

This fits with #2384 - currently #[new] assumes you want to build a new object of Self type each time, but really we should relax this restriction (just needs someone to work out the implementation).