Rustc should be able to unify `<T as A<'static>>::B>` with `<T as A<'a>>::B>` for all `'a`, given `A::B: 'static`

[Boscop]

I had to use this trick to work around the lack of working GATs:
Ended up with this code: https://play.rust-lang.org/?gist=5f14fd0600bf0b1e1e9eac2c8a3d194a&version=stable
It works but to be able to do

pub struct AppMgr<T: for<'a> DeviceApp<'a>> {
	device: Option<T::Dev>,
	app: T,
	// ...
}

I had to split up my trait

pub trait DeviceApp<'a> {
	type Dev: Device;
	// ...

into two

pub trait DeviceSpecific {
	type Dev: Device;
}

pub trait DeviceApp<'a>: DeviceSpecific {
	// ...

to pull Dev out of the quantification scope of 'a.

I thought it should work if I put Dev back inside DeviceApp but make it : 'static so that rustc can infer that it's the same for all 'a:

/*
pub trait DeviceSpecific {
	type Dev: Device;
}

pub trait DeviceApp<'a>: DeviceSpecific {
*/
pub trait DeviceApp<'a> {
	type Dev: Device + 'static;

// ...

pub struct AppMgr<T: for<'a> DeviceApp<'a>> {
	// device: Option<T::Dev>,
	device: Option<<T as DeviceApp<'static>>::Dev>, // because Dev: 'static, i thought it will be unifyable with any 'a
	app: T,
	// ...
}

But it doesn't work. Now I get this error:

error[E0495]: cannot infer an appropriate lifetime for lifetime parameter 'a in function call due to conflicting requirements
  --> src/main.rs:79:53
   |
79 |             device.frame(now, |device, input| { app.process(device, input, v) })
   |                                                     ^^^^^^^
   |
note: first, the lifetime cannot outlive the lifetime 'a as defined on the method body at 75:5...
  --> src/main.rs:75:5
   |
75 |     pub fn process<'a>(&mut self, now: u64, msgs: Vec<<T as DeviceApp<'a>>::Msg>, v: &'a mut <T as DeviceApp<'a>>::BorrowedState) -> Vec<<T as DeviceApp<'a>>::Event> {
   |     ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   = note: ...so that the expression is assignable:
           expected &mut <T as DeviceApp<'_>>::BorrowedState
              found &mut <T as DeviceApp<'a>>::BorrowedState
   = note: but, the lifetime must be valid for the static lifetime...
   = note: ...so that the expression is assignable:
           expected &mut <T as DeviceApp<'_>>::Dev
              found &mut <T as DeviceApp<'static>>::Dev

https://play.rust-lang.org/?gist=d4084c0a87890d4ed17d0491e453565b&version=stable

Why can't rustc unify <T as DeviceApp<'static>>::Dev> with <T as DeviceApp<'a>>::Dev> for all 'a, when it KNOWS that Dev: 'static (the type doesn't depend on 'a, so it can only be the same for all 'a)?

It should be able to infer it..

[steveklabnik]

Triage: no change