error: expected one of `>`, identifier, lifetime, or type, found `,`
  --> $DIR/issue-20616-7.rs:40:22
   |
LL | type Type_7 = Box<(),,>; //~ error: expected one of `>`, identifier, lifetime, or type, found `,`
   |                      ^ expected one of `>`, identifier, lifetime, or type here

error: aborting due to previous error