| A type parameter with default value is using forward declared identifier. |
| |
| Erroneous code example: |
| |
| ```compile_fail,E0128 |
| 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: |
| |
| ``` |
| 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. |
