compiler/rustc_error_codes/src/error_codes/E0128.md

   1 A type parameter with default value is using forward declared identifier.
   2
   3 Erroneous code example:
   4
   5 ```compile_fail,E0128
   6 struct Foo<T = U, U = ()> {
   7     field1: T,
   8     field2: U,
   9 }
  10 // error: type parameters with a default cannot use forward declared
  11 //        identifiers
  12 ```
  13
  14 Type parameter defaults can only use parameters that occur before them. Since
  15 type parameters are evaluated in-order, this issue could be fixed by doing:
  16
  17 ```
  18 struct Foo<U = (), T = U> {
  19     field1: T,
  20     field2: U,
  21 }
  22 ```
  23
  24 Please also verify that this wasn't because of a name-clash and rename the type
  25 parameter if so.