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# Error code E0735

Type parameter defaults cannot use `Self` on structs, enums, or unions.

Erroneous code example:

    #![allow(unused)]
    fn main() {
    struct Foo<X = Box<Self>> {
        field1: Option<X>,
        field2: Option<X>,
    }
    // error: type parameters cannot use `Self` in their defaults.
    }