Closed
Description
I was working on some generic code when I hit this bug, so I made this MRE:
use std::borrow::Borrow;
trait TraitA<'a> {
type Return;
fn get(&'a self) -> Self::Return;
}
trait TraitB<T>
where
for<'a> <Self::TrA as TraitA<'a>>::Return: Borrow<T>,
{
type TrA: for<'a> TraitA<'a>;
}
fn takes_trait_b<T: TraitB<String>>(x: T)
where
for<'a> <T::TrA as TraitA<'a>>::Return: Borrow<String>,
{
}
impl<T: 'static> TraitB<T> for T {
type TrA = Returner<T>;
}
struct Returner<T>(T);
impl<'a, T: 'static> TraitA<'a> for Returner<T> {
type Return = &'a T;
fn get(&'a self) -> Self::Return {
&self.0
}
}
fn test() {
takes_trait_b::<String>(String::new());
takes_trait_b(String::new());
}(playground link: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=ce2ea68678f61b329a6cec59310c1aea)
I'd expect both calls to takes_trait_b to succeed, but the second call doesn't as rust seemingly tries to uphold the where clause for any possible T
This has been observed on rustc 1.57.0-nightly (aa8f2d432 2021-09-18)
Adding two extra where clauses makes both calls compile on stable, but this example still fails on nightly: https://play.rust-lang.org/?version=stable&mode=debug&edition=2018&gist=aaa115e139635787f9b033a1418c7f06
Seems like a regression.