error[E0119]: conflicting implementations of trait `From<()>` for type `S`
  --> $DIR/inter-crate-ambiguity-causes-notes.rs:12:1
   |
LL |   impl From<()> for S {
   |   ------------------- first implementation here
...
LL | / impl<I> From<I> for S
LL | |
LL | | where
LL | |     I: Iterator<Item = ()>,
   | |___________________________^ conflicting implementation for `S`
   |
   = note: upstream crates may add a new impl of trait `std::iter::Iterator` for type `()` in future versions

error: aborting due to previous error

For more information about this error, try `rustc --explain E0119`.