Compiler forgets the trait bound on an associated type if the concrete type is known

I tried this code:

trait SomeTrait {}

trait Foo {
    type Assoc: SomeTrait;
}

struct Wrap<T>(T);

fn require_trait<T: SomeTrait>() {}

fn works<T: Foo>() {
    require_trait::<<T as Foo>::Assoc>();
}

fn fails<T: Foo<Assoc = Wrap<T>>>() {
    require_trait::<<T as Foo>::Assoc>();
}

I expected the code to compile, but I got the following error instead:

   Compiling playground v0.0.1 (/playground)
error[E0277]: the trait bound `Wrap<T>: SomeTrait` is not satisfied
  --> src/lib.rs:16:21
   |
16 |     require_trait::<<T as Foo>::Assoc>();
   |                     ^^^^^^^^^^^^^^^^^ the trait `SomeTrait` is not implemented for `Wrap<T>`
   |
help: this trait has no implementations, consider adding one
  --> src/lib.rs:1:1
   |
1  | trait SomeTrait {}
   | ^^^^^^^^^^^^^^^
note: required by a bound in `require_trait`
  --> src/lib.rs:9:21
   |
9  | fn require_trait<T: SomeTrait>() {}
   |                     ^^^^^^^^^ required by this bound in `require_trait`

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

Note that the works function compiles, even though it's strictly more general than the fails function.

Discovered by sarah8389 on the rust community discord.

rust-lang / rust

Compiler forgets the trait bound on an associated type if the concrete type is known #130429

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