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# Error code E0720

An `impl Trait` type expands to a recursive type.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    fn make_recursive_type() -> impl Sized {
        [make_recursive_type(), make_recursive_type()]
    }
    }

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]`.