This error indicates that the compiler is unable to determine whether there is
exactly one unique region in the set of derived region bounds.

Example of erroneous code:

```compile_fail,E0227
trait Foo<'foo>: 'foo {}
trait Bar<'bar>: 'bar {}

trait FooBar<'foo, 'bar>: Foo<'foo> + Bar<'bar> {}

struct Baz<'foo, 'bar> {
    baz: dyn FooBar<'foo, 'bar>,
}
```

Here, `baz` can have either `'foo` or `'bar` lifetimes.

To resolve this error, provide an explicit lifetime:

```rust
trait Foo<'foo>: 'foo {}
trait Bar<'bar>: 'bar {}

trait FooBar<'foo, 'bar>: Foo<'foo> + Bar<'bar> {}

struct Baz<'foo, 'bar, 'baz>
where
    'baz: 'foo + 'bar,
{
    obj: dyn FooBar<'foo, 'bar> + 'baz,
}
```