error: expected one of `>`, a const expression, lifetime, or type, found `,`
  --> $DIR/issue-20616-5.rs:22:34
   |
LL | type Type_5<'a> = Type_1_<'a, (),,>;
   |                                  ^ expected one of `>`, a const expression, lifetime, or type
   |
help: you might have meant to end the type parameters here
   |
LL | type Type_5<'a> = Type_1_<'a, ()>,,>;
   |                                 +

error: aborting due to previous error