Type annotations needed for type alias that refers to associated type on trait #105680
Description
I tried this code:
enum Foo<A> {
A(A)
}
trait Baz {
type A;
}
type Bar<B> = Foo<<B as Baz>::A>;
struct Impl {
bar: Bar<Self>
}
impl Baz for Impl {
type A = String;
}
fn main() {
let s = Impl {
bar: Bar::A(String::new())
};
}Playground link: https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=97b88d916216f04a93c1d24ca2a4f558
I expected to see this happen: It compiles without issues.
Instead, this happened:
Compiling playground v0.0.1 (/playground)
error[[E0284]](https://doc.rust-lang.org/stable/error-index.html#E0284): type annotations needed
--> src/main.rs:22:14
|
22 | bar: Bar::A(String::new())
| ^^^ cannot infer type for type parameter `B` declared on the type alias `Bar`
|
= note: cannot satisfy `<_ as Baz>::A == String`
For more information about this error, try `rustc --explain E0284`.
error: could not compile `playground` due to previous error
Unless I am missing something, the type of Impl::Bar is defined unambiguously to be Foo<String>. It works once you add the type annotation:
let s = Impl {
bar: Bar::<Impl>::A(String::new())
};Meta
The problem is reproducible for stable, beta and nightly (tried in playground).
rustc --version --verbose:
rustc 1.65.0 (897e37553 2022-11-02)
binary: rustc
commit-hash: 897e37553bba8b42751c67658967889d11ecd120
commit-date: 2022-11-02
host: x86_64-unknown-linux-gnu
release: 1.65.0
LLVM version: 15.0.0