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TAIT: it's possible to impl a trait for a tait by using projections #99840

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#103488
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TAIT: it's possible to impl a trait for a tait by using projections#99840
#103488
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F-type_alias_impl_trait`#[feature(type_alias_impl_trait)]`I-ICEIssue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️I-unsoundIssue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/SoundnessT-typesRelevant to the types team, which will review and decide on the PR/issue.requires-nightlyThis issue requires a nightly compiler in some way. When possible, use a F-* label instead.

Description

@lcnr
lcnr
opened on Jul 28, 2022
#![feature(type_alias_impl_trait)]
type Alias = impl Sized;

fn constrain() -> Alias {
    1i32
}

trait HideIt {
    type Assoc;
}

impl HideIt for () {
    type Assoc = Alias;
}

pub trait Yay {}

impl Yay for <() as HideIt>::Assoc {}
// impl Yay for i32 {} // this errors
// impl Yay for u32 {} // this also errors

As coherence checking keeps opaque types opaque this should not be unsound. Considering that a direct impl for opaque types is forbidden, this is an inconsistency we should resolve though.

cc @oli-obk

Activity

  1. lcnr
    added
    F-type_alias_impl_trait`#[feature(type_alias_impl_trait)]`
    T-typesRelevant to the types team, which will review and decide on the PR/issue.
    on Jul 28, 2022

  2. oli-obk commented on Jul 28, 2022

    @oli-obk
    oli-obk
    on Jul 28, 2022
    Contributor

    The two commented out lines do not cause any errors (https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=8366951aa5afd218745dd0ebc4564387) So I'd say this is unsound or at least causes ICEs: https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=07fbbfb3b0ecc1dc59ffc71b18b06f1e

  3. oli-obk
    added
    I-ICEIssue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️
    on Jul 28, 2022

  4. lcnr commented on Jul 28, 2022

    @lcnr
    lcnr
    on Jul 28, 2022
    ContributorAuthor

    what, is that a recent regression? :o these do error with rustc 1.63.0-nightly (5435ed691 2022-06-07)

  5. lcnr
    added
    I-unsoundIssue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/Soundness
    requires-nightlyThis issue requires a nightly compiler in some way. When possible, use a F-* label instead.
    on Jul 28, 2022
  6. oli-obk
    added
    E-needs-bisectionCall for participation: This issue needs bisection: https://github.com/rust-lang/cargo-bisect-rustc
    on Jul 28, 2022

  7. lqd commented on Jul 28, 2022

    @lqd
    lqd
    on Jul 28, 2022
    Member

    It looks to be from last week's #99506, and out of that rollup #99345 looks likely.

  8. oli-obk commented on Jul 28, 2022

    @oli-obk
    oli-obk
    on Jul 28, 2022
    Contributor

    Hmm... that one got reverted in #99079

    cc @compiler-errors

  9. lqd commented on Jul 28, 2022

    @lqd
    lqd
    on Jul 28, 2022
    Member

    Was that a revert, or a targeted fix for closures ?

  10. oli-obk commented on Jul 28, 2022

    @oli-obk
    oli-obk
    on Jul 28, 2022
    Contributor

    revert + actual fix of the original issue

  11. lqd commented on Jul 28, 2022

    @lqd
    lqd
    on Jul 28, 2022
    Member

    The behavior is the same on master, no errors + ICEs. I'll try to manually bisect within the rollup to confirm cargo-bisect-rustc's findings.

  12. compiler-errors commented on Jul 28, 2022

    @compiler-errors
    compiler-errors
    on Jul 28, 2022
    Contributor

    Isn't this just another form of #99554?

  13. lqd commented on Jul 28, 2022

    @lqd
    lqd
    on Jul 28, 2022
    Member

    (I wish we had try builds for each rolled up PR 😅) It was indeed introduced in the same rollup: by eecfdfb / #99383.

    Reverting that does indeed bring the conflicting implementation errors back, and fixes the ICE.

  14. lcnr commented on Jul 28, 2022

    @lcnr
    lcnr
    on Jul 28, 2022
    ContributorAuthor

    Isn't this just another form of #99554?

    no, #99554 is using projections in ways which should fail the orphan check, here we have projections which should pass the orphan check, but can be normalized to opaque types.

    These issues need two distinct fixes

    the fix for this being: add tests for auto trait stuff with opaque types but allow them in impls. As coherence doesn't use Reveal::All, having one impl trait ref with impl A<X, opaque, Y> for Z should conflict with all impl A<X, whatever, Y> for Z. Have to think more about this though

  15. oli-obk commented on Jul 28, 2022

    @oli-obk
    oli-obk
    on Jul 28, 2022
    Contributor

    I have a proper fix locally, but we should investigate how #99383 affected this

  16. lqd
    removed
    E-needs-bisectionCall for participation: This issue needs bisection: https://github.com/rust-lang/cargo-bisect-rustc
    on Jul 29, 2022

  17. ouz-a commented on Aug 1, 2022

    @ouz-a
    ouz-a
    on Aug 1, 2022
    Contributor

    (I wish we had try builds for each rolled up PR 😅) It was indeed introduced in the same rollup: by eecfdfb / #99383.

    Reverting that does indeed bring the conflicting implementation errors back, and fixes the ICE.

    Although it looks suspicious this is not caused by #99383 I reverted all changes made by it and tested it, this still passes without any errors. Here https://github.com/ouz-a/rust/tree/undo_formalize

  18. oli-obk
    moved this to Todo in type alias impl trait stabilizationon Sep 9, 2022
  19. oli-obk
    added this to type alias impl trait stabilizationon Sep 9, 2022
  20. oli-obk
    moved this from Todo to In Progress in type alias impl trait stabilizationon Oct 30, 2022
  21. Manishearth
    added 4 commits that reference this issue on Nov 22, 2022

    Rollup merge of rust-lang#103488 - oli-obk:impl_trait_for_tait, r=lcnr

    43eb708

    Rollup merge of rust-lang#103488 - oli-obk:impl_trait_for_tait, r=lcnr

    06666bb

    Rollup merge of rust-lang#103488 - oli-obk:impl_trait_for_tait, r=lcnr

    8c1cec9

    Rollup merge of rust-lang#103488 - oli-obk:impl_trait_for_tait, r=lcnr

    53eab24
  22. bors
    closed this as completedin 53eab24on Nov 23, 2022
  23. Repository owner moved this from In Progress to Done in type alias impl trait stabilizationon Nov 23, 2022
  24. flip1995
    added a commit that references this issue on Dec 1, 2022

    Rollup merge of rust-lang#103488 - oli-obk:impl_trait_for_tait, r=lcnr

    91d2ce3
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    F-type_alias_impl_trait`#[feature(type_alias_impl_trait)]`I-ICEIssue: The compiler panicked, giving an Internal Compilation Error (ICE) ❄️I-unsoundIssue: A soundness hole (worst kind of bug), see: https://en.wikipedia.org/wiki/SoundnessT-typesRelevant to the types team, which will review and decide on the PR/issue.requires-nightlyThis issue requires a nightly compiler in some way. When possible, use a F-* label instead.

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