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The never type can be named despite its feature gate #58733

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@SimonSapin

Reported by @xfix in #33417 (comment).

trait MyTrait {
    type Output;
}

impl<T> MyTrait for fn() -> T {
    type Output = T;
}

type Void = <fn() -> ! as MyTrait>::Output;

fn main() {
    let _a: Void;
}

The never type is unstable (#35121) but the code above (which doesn’t use any feature gate) compiles in Rust 1.12.0 (which I think is the first with #35162), 1.32.0, and 1.34.0-nightly (aadbc45 2019-02-23). In 1.11.0, it errors with:

error: the trait bound `fn() -> !: MyTrait` is not satisfied [--explain E0277]
  --> a.rs:12:13
   |>
12 |>     let _a: Void;
   |>             ^^^^
help: the following implementations were found:
help:   <fn() -> T as MyTrait>

error: aborting due to previous error

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    A-stabilityArea: `#[stable]`, `#[unstable]` etc.C-bugCategory: This is a bug.T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.

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