Compile failing even though it seems like it should work

help
1

As far as I can tell, the below code should work, but it errors:

use core::marker::PhantomData;

pub trait Value: Sized {}

pub struct Val<'ctx: 'scope, 'scope, T: Value> {
    _phantom: PhantomData<(&'scope &'ctx (), T)>,
}

pub trait AggregateValue<'ctx: 'scope, 'scope>: Value {
    type AggregateOfVals: Copy + 'scope;

    fn match_val<
        'inner_scope,
        R: Value,
        F: 'scope
            + FnMut(
                <Self as AggregateValue<'ctx, 'inner_scope>>::AggregateOfVals,
            ) -> Val<'ctx, 'inner_scope, R>,
    >(
        value: Val<'ctx, 'scope, Self>,
        f: F,
        inner_scope: &'inner_scope (),
    ) -> Val<'ctx, 'scope, R>
    where
        'scope: 'inner_scope,
        Self: AggregateValue<'ctx, 'inner_scope>;
}

(Playground)

Errors:

   Compiling playground v0.0.1 (/playground)
error[E0284]: type annotations needed: cannot satisfy `<Self as AggregateValue<'ctx, 'inner_scope>>::AggregateOfVals == _`
  --> src/lib.rs:12:5
   |
12 | /     fn match_val<
13 | |         'inner_scope,
14 | |         R: Value,
15 | |         F: 'scope
...  |
25 | |         'scope: 'inner_scope,
26 | |         Self: AggregateValue<'ctx, 'inner_scope>;
   | |_________________________________________________^ cannot satisfy `<Self as AggregateValue<'ctx, 'inner_scope>>::AggregateOfVals == _`

error[E0284]: type annotations needed: cannot satisfy `<Self as AggregateValue<'ctx, 'inner_scope>>::AggregateOfVals == <Self as AggregateValue<'ctx, 'inner_scope>>::AggregateOfVals`
  --> src/lib.rs:12:8
   |
12 |     fn match_val<
   |        ^^^^^^^^^ cannot satisfy `<Self as AggregateValue<'ctx, 'inner_scope>>::AggregateOfVals == <Self as AggregateValue<'ctx, 'inner_scope>>::AggregateOfVals`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0284`.
error: could not compile `playground`

To learn more, run the command again with --verbose.
2

Submitted a bug report: https://github.com/rust-lang/rust/issues/87208

3

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