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Can't pass GAT as FnOnce argument (interaction between GATs and HRTBs?) #85921

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elidupree opened this issue Jun 2, 2021 · 4 comments · Fixed by #89975
Closed

Can't pass GAT as FnOnce argument (interaction between GATs and HRTBs?) #85921

elidupree opened this issue Jun 2, 2021 · 4 comments · Fixed by #89975
Labels
A-associated-items Area: Associated items such as associated types and consts. A-closures Area: closures (`|args| { .. }`) C-bug Category: This is a bug. E-needs-test Call for participation: An issue has been fixed and does not reproduce, but no test has been added. F-generic_associated_types `#![feature(generic_associated_types)]` a.k.a. GATs GATs-triaged Issues using the `generic_associated_types` feature that have been triaged T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.

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@elidupree
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This code [playground]

#![feature(generic_associated_types)]

trait Trait {
  type Assoc<'a>;
  
  fn with_assoc(f: impl FnOnce(Self::Assoc<'_>));
}

impl Trait for () {
  type Assoc<'a> = i32;

  fn with_assoc(f: impl FnOnce(Self::Assoc<'_>)) {
    f(5i32)
  }
}

produces these errors:

error[E0308]: mismatched types
  --> src/lib.rs:13:7
   |
10 |   type Assoc<'a> = i32;
   |   --------------------- expected this associated type
...
13 |     f(5i32)
   |       ^^^^ expected associated type, found `i32`
   |
   = note: expected associated type `<() as Trait>::Assoc<'_>`
                         found type `i32`

error[E0277]: expected a `FnOnce<(i32,)>` closure, found `impl FnOnce(Self::Assoc<'_>)`
  --> src/lib.rs:13:5
   |
13 |     f(5i32)
   |     ^^^^^^^ expected an `FnOnce<(i32,)>` closure, found `impl FnOnce(Self::Assoc<'_>)`
   |
help: consider further restricting this bound
   |
12 |   fn with_assoc(f: impl FnOnce(Self::Assoc<'_>) + std::ops::FnOnce<(i32,)>) {
   |                                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^

in 1.54.0-nightly (2021-05-27 1c6868a)

Note that you get one of these errors even if you allow Rust to infer the type by producing a value from thin air, implying that the compiler first infers that Assoc<'_> is i32, then complains that Assoc<'_> and i32 are different.

@elidupree elidupree added the C-bug Category: This is a bug. label Jun 2, 2021
@Aaron1011
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I suspect that this will be fixed by #85499

@jackh726 jackh726 added the F-generic_associated_types `#![feature(generic_associated_types)]` a.k.a. GATs label Jun 4, 2021
@inquisitivecrystal
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inquisitivecrystal commented Jun 5, 2021

@rustbot label +A-associated-items +A-closures +T-compiler

@rustbot rustbot added A-associated-items Area: Associated items such as associated types and consts. A-closures Area: closures (`|args| { .. }`) T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Jun 5, 2021
@jackh726
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Fixed in latest nightly

@jackh726 jackh726 added the E-needs-test Call for participation: An issue has been fixed and does not reproduce, but no test has been added. label Jul 19, 2021
@jackh726
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GATs issue triage: not blocking. Works now, just needs test.

@jackh726 jackh726 added the GATs-triaged Issues using the `generic_associated_types` feature that have been triaged label Oct 16, 2021
JohnTitor added a commit to JohnTitor/rust that referenced this issue Oct 17, 2021
bors added a commit to rust-lang-ci/rust that referenced this issue Oct 17, 2021
…askrgr

Rollup of 7 pull requests

Successful merges:

 - rust-lang#89738 (ty::pretty: prevent infinite recursion for `extern crate` paths.)
 - rust-lang#89888 (Make `llvm.download-ci-llvm="if-available"` work for tier 2 targets with host tools)
 - rust-lang#89945 (Remove a mention to `copy_from_slice` from `clone_from_slice` doc)
 - rust-lang#89946 (Fix an ICE with TAITs and Future)
 - rust-lang#89963 (Some "parenthesis" and "parentheses" fixes)
 - rust-lang#89975 (Add a regression test for rust-lang#85921)
 - rust-lang#89977 (Make Result::as_mut const)

Failed merges:

r? `@ghost`
`@rustbot` modify labels: rollup
@bors bors closed this as completed in 1ee7c29 Oct 17, 2021
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Labels
A-associated-items Area: Associated items such as associated types and consts. A-closures Area: closures (`|args| { .. }`) C-bug Category: This is a bug. E-needs-test Call for participation: An issue has been fixed and does not reproduce, but no test has been added. F-generic_associated_types `#![feature(generic_associated_types)]` a.k.a. GATs GATs-triaged Issues using the `generic_associated_types` feature that have been triaged T-compiler Relevant to the compiler team, which will review and decide on the PR/issue.
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5 participants