error[E0119]: conflicting implementations of trait `From<()>` for type `S`
  --> $DIR/inter-crate-ambiguity-causes-notes.rs:9:1
   |
LL | impl From<()> for S {
   | ------------------- first implementation here
...
LL | impl<I> From<I> for S
   | ^^^^^^^^^^^^^^^^^^^^^ 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`.