Error code E0720

An impl Trait type expands to a recursive type.

Erroneous code example:

fn make_recursive_type() -> impl Sized {
    [make_recursive_type(), make_recursive_type()]
}
Run

An impl Trait type must be expandable to a concrete type that contains no impl Trait types. For example the previous example tries to create an impl Trait type T that is equal to [T, T].

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