Error code E0128

A type parameter with default value is using forward declared identifier.

Erroneous code example:

#![allow(unused)] fn main() { struct Foo<T = U, U = ()> { field1: T, field2: U, } // error: generic parameters with a default cannot use forward declared // identifiers }

Type parameter defaults can only use parameters that occur before them. Since type parameters are evaluated in-order, this issue could be fixed by doing:

#![allow(unused)] fn main() { struct Foo<U = (), T = U> { field1: T, field2: U, } }

Please also verify that this wasn't because of a name-clash and rename the type parameter if so.