error[E0004]: non-exhaustive patterns: `C(QA)` not covered
  --> $DIR/issue-40221.rs:11:11
   |
LL | / enum P {
LL | |     C(PC),
   | |     - not covered
LL | | }
   | |_- `P` defined here
...
LL |       match proto {
   |             ^^^^^ pattern `C(QA)` not covered
   |
   = note: the matched value is of type `P`
help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms
   |
LL ~         P::C(PC::Q) => (),
LL ~         C(QA) => todo!(),
   |

error: aborting due to previous error

For more information about this error, try `rustc --explain E0004`.