Bounds Inference Failure with Nested Associated Types

[preston-evans98]

I tried this code:

pub trait BaseTrait {
    type MyType;
}

pub trait BaseTraitExt: BaseTrait
where Self::MyType: std::fmt::Debug {}

pub trait Container { 
    type Extended: BaseTraitExt;
}

I expected the code to compile successfully.

Instead, compilation fails with the following error:

error[E0277]: `<<Self as Container>::Extended as BaseTrait>::MyType` doesn't implement `Debug`
 --> src/main.rs:9:20
  |
9 |     type Extended: BaseTraitExt;
  |                    ^^^^^^^^^^^^ `<<Self as Container>::Extended as BaseTrait>::MyType` cannot be formatted using `{:?}` because it doesn't implement `Debug`
  |
  = help: the trait `Debug` is not implemented for `<<Self as Container>::Extended as BaseTrait>::MyType`
note: required by a bound in `BaseTraitExt`
 --> src/main.rs:6:21
  |
5 | pub trait BaseTraitExt: BaseTrait
  |           ------------ required by a bound in this trait
6 | where Self::MyType: std::fmt::Debug {}
  |                     ^^^^^^^^^^^^^^^ required by this bound in `BaseTraitExt`
help: consider further restricting the associated type
  |
8 | pub trait Container where <<Self as Container>::Extended as BaseTrait>::MyType: Debug { 
  |                     +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++

For more information about this error, try `rustc --explain E0277`.
error: could not compile `playground` (bin "playground") due to 1 previous error

Meta

rustc --version --verbose:

rustc 1.75.0 (82e1608df 2023-12-21)
binary: rustc
commit-hash: 82e1608dfa6e0b5569232559e3d385fea5a93112
commit-date: 2023-12-21
host: aarch64-apple-darwin
release: 1.75.0
LLVM version: 17.0.6

[kadiwa4]

Very similar to #103387 and #109325

[compiler-errors]

Yes, this is a duplicate of both of these.

[preston-evans98]

In case anyone comes across this issue, here's the workaround (thanks to Eric Langlois on StackOverflow).

pub trait BaseTrait {
    type MyType;
}
pub trait MyTypeExt: core::fmt::Debug {}
trait PrivateHelper: BaseTrait<MyType = <Self as PrivateHelper>::MyType> {
    type MyType: MyTypeExt;
}
trait BaseTraitExt: PrivateHelper {}

impl<T: core::fmt::Debug> MyTypeExt for T {}
impl<T> PrivateHelper for T
where
    T: BaseTrait,
    T::MyType: MyTypeExt,
{
    type MyType = <T as BaseTrait>::MyType;
}
impl<T> BaseTraitExt for T where T: PrivateHelper {}