conflicting implementations of trait `std::ops::Drop` for type `Struct<_>` : > **TL;DR** https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=99952dfdc8dab353992d2681de6b6f58

> **Full version** https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=38d0c934cb7e55b868d73bd2dde94454

I don't quite understand why this doesn't work:
```rs
pub trait State {}
pub trait WithFinal: State {}
pub struct Machine<T: State> {
    pub state: T,
    error: Option<fn(&Event, &T)>,
    transition: Option<fn(&T, &T, Event)>, // fn(&current_state, &previous_state)
}

impl<T: WithFinal> Drop for Machine<T> {
    fn drop(&mut self) {}
}
```
```console
   Compiling scdlang v0.1.0 (/home/wildan/Projects/OSS/scdlang)
error[E0367]: The requirement `T: statechart::WithFinal` is added only by the Drop impl.
  --> src/main.rs:92:5
   |
92 | /     impl<T: WithFinal> Drop for Machine<T> {
93 | |         fn drop(&mut self) {}
94 | |     }
   | |_____^
   |
note: The same requirement must be part of the struct/enum definition
  --> src/main.rs:74:5
   |
74 | /     pub struct Machine<T: State> {
75 | |         pub state: T,
76 | |         error: Option<fn(&Event, &T)>,
77 | |         transition: Option<fn(&T, &T, Event)>, // fn(&current_state, &previous_state)
78 | |     }
   | |_____^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0367`.
error: Could not compile `scdlang`.

To learn more, run the command again with --verbose.
```
I thought it should be working because `WithFinal` extend trait `State`

However, either these 2 `impl` work just fine:
```rs
trait DropLike {
    fn drop(&mut self);
}

impl<T: WithFinal> DropLike for Machine<T> {
    fn drop(&mut self) {}
}

impl<T: State> Drop for Machine<T> {
    fn drop(&mut self) {}
}

```