Type inference fails in unpredictable ways

[notriddle]

This works

#![allow(unused)]

fn f<F>() -> Result<Option<F>, ()> {
    panic!()
}

fn g<G>() -> Option<G> {
    f().unwrap().unwrap()
}

This does not:

#![allow(unused)]

fn f<F>() -> Result<Option<F>, ()> {
    panic!()
}

fn g<G>() -> Option<G> where Option<G>: Sized { // <--
    f().unwrap().unwrap()
}

[jonas-schievink]

Might be related to #24066 (explained in more detail in this comment)

[dtolnay]

Thanks for filing this! Came up in serde-rs/serde#305.

[arielb1]

workaround: explicitly specify type parameters rather than leaving them to inference:

#![allow(unused)]

fn f<F>() -> Result<Option<F>, ()> {
    panic!()
}

fn g<G>() -> Option<G> where Option<G>: Sized {
    f::<Option<G>>().unwrap().unwrap()
}

fn main() {}

[bstrie]

Triage: the given code works when tested on Rust 1.41. Closing.