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:

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:

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,
}