error: expected one of `>`, identifier, lifetime, or type, found `,`
  --> $DIR/issue-20616-3.rs:25:24
   |
LL | type Type_3<T> = Box<T,,>; //~ error: expected one of `>`, identifier, lifetime, or type, found `,`
   |                        ^ expected one of `>`, identifier, lifetime, or type here

error: aborting due to previous error