error[E0284]: type annotations needed
  --> $DIR/question-mark-type-infer.rs:12:21
   |
LL |     l.iter().map(f).collect()?
   |                     ^^^^^^^ cannot infer type
   |
   = note: cannot satisfy `<_ as Try>::Residual == _`
help: consider specifying the type argument in the method call
   |
LL |     l.iter().map(f).collect::<B>()?
   |                            +++++

error: aborting due to previous error

For more information about this error, try `rustc --explain E0284`.