Another GAT issue: for<'a> <_ as S>::T<'a>: U<'a> is not satisfied

help
2

I like translating these lifetime-only GAT examples back into stable Rust. Let's see what we get.

struct MyDatum<'a> {
    s: &'a str,
}

trait MaybeString<'a> {
    fn get_str(&self) -> Option<&'a str>;
}

impl<'a> MaybeString<'a> for MyDatum<'a> {
    fn get_str(&self) -> Option<&'a str> {
        Some(self.s)
    }
}

trait HasDatum<'a> {
    type Datum;
}
type Datum<'a, T> = <T as HasDatum<'a>>::Datum;

trait Machine: for<'a> HasDatum<'a> {}

struct MyMachine {}

impl<'a> HasDatum<'a> for MyMachine {
    type Datum = MyDatum<'a>;
}

impl Machine for MyMachine {}

fn run<M>(m: M)
where
    M: Machine,
    for<'a> Datum<'a, M>: MaybeString<'a>,
{
}

fn main() {
    run(MyMachine {});
}

error[E0277]: the trait bound `for<'a> <_ as HasDatum<'a>>::Datum: MaybeString<'a>` is not satisf
   --> src/main.rs:38:5
    |
 38 |     run(MyMachine {});
    |     ^^^ the trait `for<'a> MaybeString<'a>` is not implemented for `<_ as HasDatum<'a>>::Datum`
    |
 note: required by a bound in `run`
   --> src/main.rs:33:27
    |
 30 | fn run<M>(m: M)
    |    --- required by a bound in this
 ...
 33 |     for<'a> Datum<'a, M>: MaybeString<'a>,
    |                           ^^^^^^^^^^^^^^^ required by this bound in `run`

wonderful, it's the same error :sweat_smile:

Let me try some things, how about implementing a helper trait for this higher-ranked bound? [short failed attempt to even reasonably do this elided…]

That's curious, the error message consistently uses _ instead of MyMachine. Does it change if we more explicitly specify the type in the call?

fn main() {
    run::<MyMachine>(MyMachine {});
}

:sweat_smile: aaaand... now it compiles. (With the GAT version, too.) Don't ask me why, feels like a bug to me that this additional information makes a difference here.

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