src/librustc_error_codes/error_codes/E0128.md

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