Introducing a GAT parameter leads to a false positive E0207 for another parameter constrained by Fn type · Issue #88526 · rust-lang/rust (original) (raw)

I tried this code:

#![feature(generic_associated_types)]

trait A { type I<'a>; }

pub struct TestA { f: F, }

impl A for TestA { type I<'a> = &'a F; }

struct TestB<Q, F> { q: Q, f: F, }

impl<'q, Q, I, F> A for TestB<Q, F> where Q: A<I<'q> = &'q I>, F: Fn(I), { type I<'a> = (); }

I expected it to compile. Instead, I've got the following error:

error[E0207]: the type parameter `I` is not constrained by the impl trait, self type, or predicates
  --> src/lib.rs:22:13
   |
22 | impl<'q, Q, I, F> A for TestB<Q, F>
   |             ^ unconstrained type parameter

For more information about this error, try `rustc --explain E0207`.
error: could not compile `playground` due to previous error

If I remove the GAT, the code compiles successfully. I've created the following two playground examples to play around with (the first one compiles, the second one doesn't):
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=15548086c5da59870ffbf5b4fe4de27a
https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=725c4e36f367bd3b1ba30917cfc38b65

I'm not 100% sure that this is a bug, but the fact, that adding a GAT parameter to (as it seems) an unrelated type produces such an error, looks strange to me.

Introducing a GAT parameter leads to a false positive E0207 for another parameter constrained by Fn type · Issue #88526 · rust-lang/rust (original) (raw)

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