error[E0282]: type annotations needed for the closure `fn((), ()) -> Result<(), _>`
  --> $DIR/cannot-infer-closure.rs:3:15
   |
LL |         Err(a)?;
   |               ^ cannot infer type of error for `?` operator
   |
   = note: `?` implicitly converts the error value into a type implementing `From<()>`
help: give this closure an explicit return type without `_` placeholders
   |
LL |     let x = |a: (), b: ()| -> Result<(), _> {
   |                            ^^^^^^^^^^^^^^^^

error: aborting due to previous error

For more information about this error, try `rustc --explain E0282`.