trait alias dyn type equality doesn't work #55629

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1. nikomatsakis

   added

   A-trait-systemArea: Trait systemArea: Trait system

   T-compilerRelevant to the compiler team, which will review and decide on the PR/issue.Relevant to the compiler team, which will review and decide on the PR/issue.

   on Nov 2, 2018
2. alexreg commented on Nov 2, 2018

   

   alexreg

   on Nov 2, 2018

   Last edited by alexreg

   Contributor

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   Here's an example from my comment in #55101:

   #![feature(optin_builtin_traits)]
   #![feature(trait_alias)]
   
   trait A<T: Send> {}
   trait B<T: Send> = A<T>;
   
   struct Foo<T>(T);
   struct Bar();
   
   impl<T: Send> A<T> for Foo<T> {}
   
   impl !Send for Bar {}
   
   fn main() {
       let b: Box<B<Bar>> = Box::new(Foo(Bar()));
       let a: Box<A<Bar>> = b; // should work
   }

   yields

   error[E0308]: mismatched types
     --> foo.rs:16:26
      |
   16 |     let a: Box<A<Bar>> = b;
      |                          ^ expected trait `A`, found trait `B`
      |
      = note: expected type `std::boxed::Box<dyn A<Bar>>`
                 found type `std::boxed::Box<dyn B<Bar>>`
   

   Whereas references to A and B should clearly be interchangeable.
3. alexreg commented on Nov 2, 2018

   

   alexreg

   on Nov 2, 2018

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   As @nikomatsakis said on Zulip, this is sort of a special case of upcasting. In general, if trait A is a subtype of trait B (i.e. A's bounds include all of B's bounds and possibly more), then dyn A should be convertible to dyn B.
4. alexreg commented on Dec 5, 2018

   

   alexreg

   on Dec 5, 2018

   Contributor

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   Note a seemingly surprising consequence on coherence that this has:

   Playground

   #![feature(trait_alias)]
   
   trait Foo {
       fn foo(&self) -> u32;
   }
   
   trait SendAlias = Send;
   
   impl Foo for dyn Send {
       fn foo(&self) -> u32 {
           1
       }
   }
   
   impl Foo for dyn SendAlias {
       fn foo(&self) -> u32 {
           2
       }
   }
   
   fn main() {
       let x1 = Box::new(()) as Box<Send>;
       let x2 = Box::new(()) as Box<SendAlias>;
       println!("{}", x1.foo());
       println!("{}", x2.foo());
   }

   This is correct behaviour for now, and it doesn't cause any ICEs, but obviously is not what one would expect intuitively.
5. Enselic

   added

   C-bugCategory: This is a bug.Category: This is a bug.

   on Nov 18, 2023
6. maltuned commented on Apr 4, 2026

   

   maltuned

   on Apr 4, 2026

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   Has this issue been fixed now?

   #![feature(trait_alias)]
   
   trait A {}
   
   trait B = A;
   
   struct S;
   
   impl A for S {}
   
   fn f(_: &dyn B) {}
   
   fn main() {
       let s = S;
       f(&s);
   }

   The above code passes compilation on rustc 1.95.0-nightly (efc9e1b50 2026-02-06).
7. tgross35

   added

   F-trait_alias`#![feature(trait_alias)]``#![feature(trait_alias)]`

   on Aug 10, 2026
8. fmease commented on Oct 22, 2026

   

   fmease

   on Oct 22, 2026

   Member

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   Has this issue been fixed now?

   Yep, nowadays we expand trait aliases "away" "eagerly" inside trait object types. Whether that behavior will stay is a separate question.
9. fmease

   closed this as completedon Oct 22, 2026
10. fmease

    added

    A-dyn-traitArea: trait objects, vtable layoutArea: trait objects, vtable layout

    T-typesRelevant to the types team, which will review and decide on the PR/issue.Relevant to the types team, which will review and decide on the PR/issue.

    on Oct 22, 2026