Bounds on associated types of super-traits are not implied

[jonhoo]

Let me first qualify this with: I'm not 100% sure this is a bug — it could be that this is simply unsupported at the moment, and that it is therefore a feature request instead.

I tried this code:

trait Bar {
    type Baz;
}

trait Foo: Bar where Self::Baz: Send {}

struct X<C>(C);
impl<C> X<C> where C: Foo {}

I expected to see this happen: the code compiles.

Instead, this happened: compilation fails with

error[E0277]: `<C as Bar>::Baz` cannot be sent between threads safely
 --> src/lib.rs:8:23
  |
5 | trait Foo: Bar where Self::Baz: Send {}
  |                                 ---- required by this bound in `Foo`
...
8 | impl<C> X<C> where C: Foo {}
  |                       ^^^ `<C as Bar>::Baz` cannot be sent between threads safely
  |
  = help: the trait `Send` is not implemented for `<C as Bar>::Baz`
help: consider further restricting the associated type
  |
8 | impl<C> X<C> where C: Foo, <C as Bar>::Baz: Send {}
  |                          ^^^^^^^^^^^^^^^^^^^^^^^

error: aborting due to previous error

My hope was that the C: Foo bound would imply the <C as Bar>::Baz: Send bound from Foo, but that does not appear to be the case. It does however work if no associated types are involved:

trait Bar2 {
}

trait Foo2: Bar2 where Self: Send {}

struct X2<C>(C);
impl<C> X2<C> where C: Foo2 {}

Which is what lead me to the belief that this is a bug (or at least "missing smartness").

The reason I want this construction in the first place is to construct what is effectively a trait bound alias for an impl block in my code that has a fairly hairy bound otherwise.

Meta

rustc --version --verbose:

rustc 1.52.1 (9bc8c42bb 2021-05-09)
binary: rustc
commit-hash: 9bc8c42bb2f19e745a63f3445f1ac248fb015e53
commit-date: 2021-05-09
host: x86_64-apple-darwin
release: 1.52.1
LLVM version: 12.0.0

The same issue arises on nightly. I also checked 1.48.0 in case #79904 was related, but 1.48.0 also fails to compile.

[jonas-schievink]

This is a duplicate of #20671