Rust: Unrecognized associated type bound on another associated type
trait Bar<T> { fn dummy(&self); }
trait Car<T> { fn dummy(&self); }
trait Foo {
type A;
type B: Bar<Self::A>;
type C: Car<Self::A>;
fn get_b(&self) -> &Self::B;
}
fn test_bar<A, B: Bar<A>>(_: &B) {}
fn test<A, F: Foo<A=A>>(f: &F) {
test_bar(f.get_b());
}
Gives me:
<anon>:15:16: 15:23 error: the trait `Bar<A>` is not implemented for the type `<F as Foo>::B` [E0277]
<anon>:15 test_bar(f.get_b());
^~~~~~~
<anon>:15:16: 15:23 error: the trait `Car<A>` is not implemented for the type `<F as Foo>::C` [E0277]
<anon>:15 test_bar(f.get_b());
^~~~~~~
error: aborting due to 2 previous errors
playpen: application terminated with error code 101
There is a workaround:
fn test<A, B: Bar<A>, C: Car<A>, F: Foo<A=A, B=B, C=C>>(f: &F) {
test_bar(f.get_b());
}
But it's ugly and should not be necessary.
We hit this problem a lot with gfx::Device, and I'd like to see cleaner use of it without explicit CommandBuffer bounds that we use as a workaround.
kvark
All 12 comments
This may be related to #23481.
apasel422
on 7 Apr 2015
Edited original post to remove PhantomFn bounds, they're no longer needed. Issue still reproduces:
error[E0277]: the trait bound `<F as Foo>::B: Bar<A>` is not satisfied
--> test.rs:14:1
|
14 | / fn test<A, F: Foo<A=A>>(f: &F) {
15 | | test_bar(f.get_b());
16 | | }
| |_^ the trait `Bar<A>` is not implemented for `<F as Foo>::B`
|
= help: consider adding a `where <F as Foo>::B: Bar<A>` bound
= note: required by `Foo`
error[E0277]: the trait bound `<F as Foo>::C: Car<A>` is not satisfied
--> test.rs:14:1
|
14 | / fn test<A, F: Foo<A=A>>(f: &F) {
15 | | test_bar(f.get_b());
16 | | }
| |_^ the trait `Car<A>` is not implemented for `<F as Foo>::C`
|
= help: consider adding a `where <F as Foo>::C: Car<A>` bound
= note: required by `Foo`
error: aborting due to 2 previous errors
Mark-Simulacrum
on 16 May 2017
A lot of related issues for this have been opened over time.
Duplicates: #37808, #39532, #40093, #48674, #54375, #54844
Probably same underlying cause: #32722, #37883, #38917, #43475, #47897, #54149
There's also a related discussion on the Rust forums: https://users.rust-lang.org/t/bounds-on-associated-types-enforced-too-soon/12948
rkarp
on 10 Dec 2017
Mildly simplified:
trait Bar<T> { fn dummy(&self); }
trait Foo {
type A;
type B: Bar<Self::A>;
fn get_b(&self) -> &Self::B;
}
fn test_bar<A, B: Bar<A>>(_: &B) {}
fn test<A, F: Foo<A=A>>(f: &F) {
test_bar(f.get_b());
}
fn main() { }
So, probably the fix is "just" that we need to do some normalization somewhere or other. My preference is to fix this by moving over to lazy normalization and chalk, which will do away with these sorts of shortcomings for good. But it's probably worth looking to see if this can be fixed a touch more rapidly by adding a normalization in the right place. I'll try to do that in a bit and mentor/open a fix if it seems reasonable.
nikomatsakis
on 13 Mar 2018
Any progress here @nikomatsakis?
malbarbo
on 20 Jun 2018
Looks like the issue is known and mentioned at this line: https://github.com/rust-lang/rust/blob/master/src/librustc_typeck/check/compare_method.rs#L252
This indicates this may be the area to check in when working on this? Still working on understanding a lot of this code, so I'm not sure.
ZerothLaw
on 17 Aug 2018
Ran into this as well. Here's my minimal test case:
trait Bar<T> {}
trait Caz {
type A;
type B: Bar<Self::A>;
}
fn test<T, U>() where T: Caz, U: Caz<A = T::A> {}
error[E0277]: the trait bound `<U as Caz>::B: Bar<<T as Caz>::A>` is not satisfied
--> src/main.rs:7:1
|
7 | fn test<T, U>() where T: Caz, U: Caz<A = T::A> {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ the trait `Bar<<T as Caz>::A>` is not implemented for `<U as Caz>::B`
|
= help: consider adding a `where <U as Caz>::B: Bar<<T as Caz>::A>` bound
note: required by `Caz`
--> src/main.rs:2:1
|
2 | trait Caz {
| ^^^^^^^^^
daboross
on 26 Aug 2018
I've just run into this issue in one of my own projects, and reduced it to a test case that's even simpler than the last one:
trait Bar<T> {}
trait Caz {
type A;
type B: Bar<Self::A>;
}
fn test<T: Caz<A = ()>>() {}
Although it compiles successfully if you add the bizarrely-tautological-looking where T::B: Bar<T::A>.
For now, I'm stuck with adding the where clause, but that's putting a LOT of boilerplate in my code (the actual where clause is 71 characters long and appears in dozens of places, including being de-facto required in code that uses my library, not just inside the library itself). Is there any progress on this?
elidupree
on 26 Mar 2019
error[E0277]: the trait bound `<F as Foo>::B: Bar<A>` is not satisfied
--> src/lib.rs:14:1
|
4 | trait Foo {
| --------- required by `Foo`
...
14 | fn test<A, F: Foo<A=A>>(f: &F) {
| ^ - help: consider further restricting the associated type: `where <F as Foo>::B: Bar<A>`
| _|
| |
15 | | test_bar(f.get_b());
16 | | }
| |_^ the trait `Bar<A>` is not implemented for `<F as Foo>::B`
error[E0277]: the trait bound `<F as Foo>::C: Car<A>` is not satisfied
--> src/lib.rs:14:1
|
4 | trait Foo {
| --------- required by `Foo`
...
14 | fn test<A, F: Foo<A=A>>(f: &F) {
| ^ - help: consider further restricting the associated type: `where <F as Foo>::C: Car<A>`
| _|
| |
15 | | test_bar(f.get_b());
16 | | }
| |_^ the trait `Car<A>` is not implemented for `<F as Foo>::C`
Applying the suggestions makes the code compile.
Edit: updated output:
error[E0277]: the trait bound `<F as Foo>::B: Bar<A>` is not satisfied
--> src/lib.rs:14:15
|
4 | trait Foo {
| --- required by a bound in this
5 | type A;
6 | type B: Bar<Self::A>;
| ------------ required by this bound in `Foo`
...
14 | fn test<A, F: Foo<A=A>>(f: &F) {
| ^^^^^^^^ the trait `Bar<A>` is not implemented for `<F as Foo>::B`
|
help: consider further restricting the associated type
|
14 | fn test<A, F: Foo<A=A>>(f: &F) where <F as Foo>::B: Bar<A> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
error[E0277]: the trait bound `<F as Foo>::C: Car<A>` is not satisfied
--> src/lib.rs:14:15
|
4 | trait Foo {
| --- required by a bound in this
...
7 | type C: Car<Self::A>;
| ------------ required by this bound in `Foo`
...
14 | fn test<A, F: Foo<A=A>>(f: &F) {
| ^^^^^^^^ the trait `Car<A>` is not implemented for `<F as Foo>::C`
|
help: consider further restricting the associated type
|
14 | fn test<A, F: Foo<A=A>>(f: &F) where <F as Foo>::C: Car<A> {
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^
error[E0277]: the trait bound `<F as Foo>::B: Bar<A>` is not satisfied
--> src/main.rs:12:1
|
3 | trait Foo {
| --------- required by `Foo`
...
12 | fn test<A, F: Foo<A=A>>(f: &F) {
| ^ - help: consider further restricting the associated type: `where <F as Foo>::B: Bar<A>`
| _|
| |
13 | | test_bar(f.get_b());
14 | | }
| |_^ the trait `Bar<A>` is not implemented for `<F as Foo>::B`
@daboross' case:
error[E0277]: the trait bound `<U as Caz>::B: Bar<<T as Caz>::A>` is not satisfied
--> src/lib.rs:7:1
|
2 | trait Caz {
| --------- required by `Caz`
...
7 | fn test<T, U>() where T: Caz, U: Caz<A = T::A> {}
| ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^-^^
| | |
| | help: consider further restricting the associated type: `, <U as Caz>::B: Bar<<T as Caz>::A>`
| the trait `Bar<<T as Caz>::A>` is not implemented for `<U as Caz>::B`
Update: this case now compiles.
In call cases applying the suggestions fulfill the requirements.
estebank
on 5 Nov 2019
@nikomatsakis any update on this? Is there anything I can do help with this? I'd be happy and try to help where I can, as this is blocking a lot of nice usages of typenum for me at the moment.
dignifiedquire
on 14 Jun 2020
It looks like this compiles under -Z chalk, or at least the bounds compile. The body function complains about inference, but I'm inclined to think that's just a chalk bug / missing feature and unrelated to this issue.
Given that, the solution might just be fully stable chalk integration. If it is, then this issue would fall into the same bucket as features like implied bounds, where there isn't a lot we can do outside of just pushing chalk integration.
daboross
on 14 Jun 2020
I think my original analysis is still correct. I think the normalizing is missing here:
After we invoke instantiate, we should also be normalizing, probably kind of like this
If Chalk solves this, it is probably because of lazy normalization.
nikomatsakis
on 15 Jun 2020
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Most helpful comment
Mildly simplified:
So, probably the fix is "just" that we need to do some normalization somewhere or other. My preference is to fix this by moving over to lazy normalization and chalk, which will do away with these sorts of shortcomings for good. But it's probably worth looking to see if this can be fixed a touch more rapidly by adding a normalization in the right place. I'll try to do that in a bit and mentor/open a fix if it seems reasonable.