Unable to deduce associated type equality

[brennie]

I'm working with combine and rust nightly to make some parsers and I'm wondering why both of these examples are not equivalent:

The first example compiles successfully:

pub fn source<'a, I>() -> impl Parser<Input = I, Output = Source>
where
    I: RangeStream<Item = u8, Range = &'a [u8]>,
    I::Error: ParseError<u8, &'a [u8], I::Position>,
    <I::Error as ParseError<I::Item, I::Range, I::Position>>::StreamError: From<str::Utf8Error>
{
// ...
}

whereas this one

pub fn source<'a, I>() -> impl Parser<Input = I, Output = Source>
where
    I: RangeStream<Item = u8, Range = &'a [u8]>,
    I::Error: ParseError<I::Item, I::Range, I::Position>,
    <I::Error as ParseError<I::Item, I::Range, I::Position>>::StreamError: From<str::Utf8Error>
{
// ...
}

produces this error:

error[E0277]: the trait bound `<I as combine::StreamOnce>::Error: combine::ParseError<u8, &'a [u8], <I as combine::StreamOnce>::Position>` is not satisfied
  --> src/parser.rs:57:1
   |
57 | / pub fn source<'a, I>() -> impl Parser<Input = I, Output = Source>
58 | | where
...  |
89 | |     )
90 | | }
   | |_^ the trait `combine::ParseError<u8, &'a [u8], <I as combine::StreamOnce>::Position>` is not implemented for `<I as combine::StreamOnce>::Error`
   |
   = help: consider adding a `where <I as combine::StreamOnce>::Error: combine::ParseError<u8, &'a [u8], <I as combine::StreamOnce>::Position>` bound

error[E0277]: the trait bound `<I as combine::StreamOnce>::Error: combine::ParseError<u8, &'a [u8], <I as combine::StreamOnce>::Position>` is not satisfied
  --> src/parser.rs:57:1
   |
57 | / pub fn source<'a, I>() -> impl Parser<Input = I, Output = Source>
58 | | where
...  |
89 | |     )
90 | | }
   | |_^ the trait `combine::ParseError<u8, &'a [u8], <I as combine::StreamOnce>::Position>` is not implemented for `<I as combine::StreamOnce>::Error`

AFAICT, the bounds: I::Error: ParseError<u8, &'a [u8], I::Position> and I::Error: ParseError<I::Item, I::Range, I::Position> should be identical since we know I::Item = u8 and I::Range = &'a [u8].

Is this a limitation of the compiler's inference, a bug, or something else?

[Marwes]

@brennie I believe this is #43475 (and a few other issues linked from that post). I think this is planned to be fixed with the trait system rewrite (lazy normalization) but I don't know what the timeline is for that :(.

[rkarp]

The oldest related issue seems to be #24159, which was even before Rust 1.0. I think fixing this should be a priority considering that many people have run into this over almost 3 years.

[jackh726]

I'm going to close this since it may be fixed already, but regardless doesn't have an MCVE. There are cases where we'll still run into these types of problems, but I know there are other issues covering more complex cases.