Subclassing doesn't work - "the trait bound T: CppPeerConstructor<TCpp> is not satisfied"

[Supreeeme]

Describe the bug
Subclassing does not compile, with the following error:

error[E0277]: the trait bound `Concrete: CppPeerConstructor<ConcreteCpp>` is not satisfied
   --> src/main.rs:12:12
    |
12  | pub struct Concrete {}
    |            ^^^^^^^^ the trait `CppPeerConstructor<ConcreteCpp>` is not implemented for `Concrete`

To Reproduce
Cargo.toml:

[package]
name = "subclass_test"
version = "0.1.0"
edition = "2021"

[dependencies]
autocxx = "0.27.0"
cxx = "1.0.124"

[build-dependencies]
autocxx-build = "0.27.0"

build.rs:

fn main() {
    autocxx_build::Builder::new("src/main.rs", ["src"])
        .build()
        .unwrap()
        .compile("test");

    println!("cargo:rerun-if-changed=src/main.rs");
    println!("cargo:rerun-if-changed=src/abstract.h");
}

main.rs:

use autocxx::prelude::*;
use autocxx::subclass::prelude::*;

include_cpp! {
    #include "abstract.h"
    generate!("Abstract")
    subclass!("Abstract", Concrete)
}

#[subclass]
#[derive(Default)]
pub struct Concrete {}

impl ffi::Abstract_methods for Concrete {
    unsafe fn func(&self) {}
}

abstract.h:

#pragma once

class Abstract {
public:
	virtual void func() const = 0;
};

Expected behavior
Successful compilation

Additional context
I'm not sure what's going on here - I can compile and run the subclassing example in the repo successfully, but trying to do it from a clean project just does not work.

[adetaylor]

I'm afraid I'm not going to have a chance to dig into this - please do gradually bisect differences relative to the demo. Subclass support is fairly fragile and the diagnostics aren't great - I'd be pleased to merge PRs to improve diagnostics here. (Perhaps the new #[diagnostic::on_unimplemented] can be used)

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