error: must return Self::Output
 --> tests/ui/fail/fn-missing-output-01.rs:7:5
  |
7 |     fn foo() {}
  |     ^^^^^^^^^^^

error: since Output is defined here
 --> tests/ui/fail/fn-missing-output-01.rs:5:19
  |
5 |     type Output = i32;
  |                   ^^^