error[E0004]: non-exhaustive patterns: `(None, None)` not covered
  --> $DIR/issue-2111.rs:2:9
   |
LL |   match (a,b) {
   |         ^^^^^ pattern `(None, None)` not covered
   |
   = help: ensure that all possible cases are being handled, possibly by adding wildcards or more match arms

error: aborting due to previous error

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