Skip to content
New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Implied bounds don't work for traits on &Self #47670

Closed
jpetkau opened this issue Jan 22, 2018 · 4 comments
Closed

Implied bounds don't work for traits on &Self #47670

jpetkau opened this issue Jan 22, 2018 · 4 comments
Labels
A-traits Area: Trait system C-enhancement Category: An issue proposing an enhancement or a PR with one. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

Comments

@jpetkau
Copy link

jpetkau commented Jan 22, 2018

If a trait has a bound on &Self, it doesn't seem to work as an implied bound.

For example (also https://gist.github.com/671c7746b96c86f9eb0e39176325157a):

use std::ops::Add;

pub trait Numoid where
  Self: Sized,
  for<'a> &'a Self: Add<Self, Output = Self>
{}

impl<N> Numoid for N where
  for<'a> &'a N: Add<N, Output = N>
{}

pub fn compute<N: Numoid>(a: N, b: N) -> N
    // where for<'a> &'a N: Add<N, Output = N> // redundant bound is required
{ &a + b }

This produces:

error[E0277]: the trait bound `for<'a> &'a N: std::ops::Add<N>` is not satisfied
  --> src/main.rs:20:1
   |
20 | / pub fn compute<N>(a: N, b: N) -> N
21 | | where
22 | |     N: Numoid,
23 | |     // for<'a> &'a N: Add<N, Output = N>, // redundant bound is required
...  |
27 | |     b + d
28 | | }
   | |_^ no implementation for `&'a N + N`
   |
   = help: the trait `for<'a> std::ops::Add<N>` is not implemented for `&'a N`
   = help: consider adding a `where for<'a> &'a N: std::ops::Add<N>` bound
   = note: required by `Numoid`

It works if the 'where' on fn compute is uncommented. Other implicit bounds in the same trait also work (e.g. for<'a> Self: Add<&'a Self> is fine).

This comes up when trying to write generic numeric code. I want to be explicit about using moves/refs to avoid unnecessary clones of things like BigInt, which requires binary operators to have the full set of overloads (i.e. a+b, &a+b, a+&b, &a+&b). BigInt and other numeric types support all these, and they work as explicit bounds, but repeating them on every function gets unwieldy quickly.

@ExpHP
Copy link
Contributor

ExpHP commented Jan 23, 2018

I believe this should eventually be addressed by the accepted Implied bounds RFC (tracking issue), which prescribes that all where bounds on traits will become implied.

@jpetkau
Copy link
Author

jpetkau commented Jan 24, 2018

Ah, I thought that RFC was already implemented, and this was a bug. I guess it's just not done yet?

@ExpHP
Copy link
Contributor

ExpHP commented Jan 24, 2018

Not much was said about it during the impl period, at least not that I can find. I added a note about what appears to be the current progress to the tracking issue, though somebody in the know may have more to add.

@pietroalbini pietroalbini added C-enhancement Category: An issue proposing an enhancement or a PR with one. A-traits Area: Trait system T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Feb 6, 2018
@jonas-schievink
Copy link
Contributor

Closing in favor of #44491 / #20671

Sign up for free to join this conversation on GitHub. Already have an account? Sign in to comment
Labels
A-traits Area: Trait system C-enhancement Category: An issue proposing an enhancement or a PR with one. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
Projects
None yet
Development

No branches or pull requests

4 participants