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Same trait with different input concrete parameters are considered duplicate bounds (: Bound<i8> + Bound<u8>) #22279
Description
STR
#![crate_type = "lib"]
pub trait Bound<T> {}
pub trait Trait: Bound<i8> + Bound<u8> {}
//~^ error: trait `Bound<u8>` already appears in the list of bounds
fn foo<T: Bound<i8> + Bound<u8>>() {}
//~^ error: trait `Bound<u8>` already appears in the list of bounds
fn foo<T>() where T: Bound<i8> + Bound<u8> {} // OKDue to multidispatch : Bound<i8> + Bound<u8> is a valid bound, but the compiler is rejecting it in some positions (supertraits and bounds in parameter list), but not in others (where clauses)
Workaround for the supertrait
You can use dummy intermediate traits to indirectly add the "duplicate" bounds, but this is very verbose.
pub trait Bound<T> {}
pub trait Dummy: Bound<i8> {}
pub trait Trait: Dummy + Bound<u8> {} // `Trait` implies `Bound<i8> + Bound<u8>`Version
rustc 1.0.0-nightly (3ef8ff1f8 2015-02-12 00:38:24 +0000)
Originally reported in #18693 (Niko preferred opening a new issue, instead of re-opening the old one and modifying the original report)
Activity
nikomatsakis commentedon Feb 13, 2015
This was an intentional limitation, but it really doesn't make a lot of sense in a multidispatch world, and is also often something you can workaround with where clauses.
jroesch commentedon Feb 14, 2015
@nikomatsakis it seems like if we commit to the supertrait/bounds changes that I started this should no longer be an issue since all bounds end up being checked by a single code path. Just an observation.
nikomatsakis commentedon Mar 3, 2015
I've wound up fixing this as part of another patch (partly due to what @jroesch was saying).
jroesch commentedon Mar 4, 2015
👍