Slightly confusing error message when using where with GATs

[AidanConnelly]

I tried this code https://play.rust-lang.org/?version=nightly&mode=debug&edition=2018&gist=374040a5ff329e636d37acd10f828a69:

#![feature(generic_associated_types)]

use either::Either;

pub trait HasChildrenOf {
    type T;
    type TRef<'a>;

    fn ref_children<'a>(&'a self) -> Vec<Self::TRef<'a>>;
    fn take_children(self) -> Vec<Self::T>;
}

impl<Left, Right> HasChildrenOf for Either<Left, Right>
where
    Left: HasChildrenOf,
    Right: HasChildrenOf,
{
    type T = Either<Left::T, Right::T>;
    type TRef<'a>
    where 
    <Left as HasChildrenOf>::T: 'a,
    <Right as HasChildrenOf>::T: 'a
    = Either<&'a Left::T, &'a Right::T>;

    fn ref_children<'a>(&'a self) -> Vec<Self::TRef<'a>> {
        todo!()
    }

    fn take_children(self) -> Vec<Self::T> {
        todo!()
    }
}

I expected to see this happen: A slightly better error message

Instead, this happened: A slightly confusing error message

error[E0309]: the associated type `<Left as HasChildrenOf>::T` may not live long enough
  --> src/lib.rs:19:5
   |
19 | /     type TRef<'a>
20 | |     where 
21 | |     <Left as HasChildrenOf>::T: 'a,
22 | |     <Right as HasChildrenOf>::T: 'a
23 | |     = Either<&'a Left::T, &'a Right::T>;
   | |________________________________________^
   |
   = help: consider adding an explicit lifetime bound `<Left as HasChildrenOf>::T: 'a`...
   = note: ...so that the type `<Left as HasChildrenOf>::T` will meet its required lifetime bounds

Meta

rustc --version --verbose:

rustc 1.55.0-nightly (868c702d0 2021-06-30)
binary: rustc
commit-hash: 868c702d0c9a471a28fb55f0148eb1e3e8b1dcc5
commit-date: 2021-06-30
host: x86_64-pc-windows-gnu
release: 1.55.0-nightly
LLVM version: 12.0.1

Backtrace