Trait bounds of associated types in where clauses result in unsatisfied requeriments

[c410-f3r]

Feel free to close if duplicated.

pub trait Foo<TY> {
    type FooAssociatedTy;
}

pub trait Bar {
    type BarAssociatedTy;
    
    fn send<T>()
    where
        T: Foo<Self::BarAssociatedTy>,
        T::FooAssociatedTy: Send;
}

impl Bar for () {
    type BarAssociatedTy = ();

    fn send<T>()
    where
        T: Foo<()>,
        T::FooAssociatedTy: Send,
    {
    }
}

Taking aside the errors, a diagnostic suggests adding a weird T: Foo<()> + Foo<()> bound.

Things that will make everything compile:

• Remove T::FooAssociatedTy: Send (https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=e67dbf2eb726ee22d3470f2a082b1980)
• Remove TY (https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=3b8275f7670e6a82dc9cd459815e66a9)
• Remove type BarAssociatedTy (https://play.rust-lang.org/?version=nightly&mode=debug&edition=2021&gist=745e08b27876ac261ad2bf9178b2cc79)

[fmease]

Fixed in the next solver (-Ztrait-solver=next).
Very similar to #41118, #108933 and #116377.

[c410-f3r]

Thanks! :)

[lcnr]

closing as dup of #41118, see the explanation in #41118 (comment)