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ICE: never type as default associated type #51506
Description
This code generates an ICE
#![feature(never_type, specialization)]
#![allow(incomplete_features)]
use std::iter::{self, Empty};
trait Trait {
type Out: Iterator<Item = u32>;
fn f(&self) -> Option<Self::Out>;
}
impl<T> Trait for T {
default type Out = !;
default fn f(&self) -> Option<Self::Out> {
None
}
}
struct X;
impl Trait for X {
type Out = Empty<u32>;
fn f(&self) -> Option<Self::Out> {
Some(iter::empty())
}
}
fn f<T: Trait>(a: T) {
if let Some(iter) = a.f() {
println!("Some");
for x in iter {
println!("x = {}", x);
}
}
}
pub fn main() {
f(10);
}with this message.
error: internal compiler error: librustc/traits/codegen/mod.rs:169: Encountered errors `[FulfillmentError(Obligation(predicate=Binder(TraitPredicate(<! as std::iter::Iterator>)),depth=1),Unimplemented)]` resolving bounds after type-checking
thread 'main' panicked at 'Box<Any>', librustc_errors/lib.rs:499:9
note: Run with `RUST_BACKTRACE=1` for a backtrace.
error: aborting due to previous error
Replacing 1.f() by X.f() makes the code to compile and run fine, which is strange because ! does not implements Iterator .
By removing the specialization the compiler refuses to compile the code with the error:
error[E0277]: the trait bound `!: std::iter::Iterator` is not satisfied
--> src/main.rs:10:9
|
10 | impl<T> Trait for T {
| ^^^^^ `!` is not an iterator; maybe try calling `.iter()` or a similar method
|
= help: the trait `std::iter::Iterator` is not implemented for `!`
error: aborting due to previous error
For more information about this error, try `rustc --explain E0277`.
Activity
JohnTitor commentedon Nov 13, 2019
The ICE has been fixed in the latest nightly, marked as E-needstest
malbarbo commentedon Nov 13, 2019
Thanks @JohnTitor. Indeed, the example compiles fine in nightly (4f03f4a 2019-11-12), however, replacing the main function by these two
generates the same ICE.
JohnTitor commentedon Nov 13, 2019
@malbarbo ah, thanks for the update! I confirmed it (playground)
JohnTitor commentedon Jun 22, 2020
Triage: the ICE doesn't occur anymore with the latest nightly, marking as E-needs-test.
Rollup merge of rust-lang#73646 - JohnTitor:add-tests, r=Dylan-DPC
Rollup merge of rust-lang#73646 - JohnTitor:add-tests, r=Dylan-DPC
Rollup merge of rust-lang#73646 - JohnTitor:add-tests, r=Dylan-DPC