BUG: setitem with mixed-resolution dt64s
- [x] closes #56410 (Replace xxxx with the GitHub issue number)
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [ ] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [ ] Added [type annotations](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#type-hints) to new arguments/methods/functions.
- [x] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

what happens after this PR for the cases in https://github.com/pandas-dev/pandas/issues/56410 ? would they emit a warning now, and raise in the future? > what happens after this PR for the cases in https://github.com/pandas-dev/pandas/issues/56410 ? would they emit a warning now, and raise in the future?

The `__setitem__` case issues a PDEP6 warning and will raise when that is enforced.  The .where and .fillna cases case to "M8[ms]". This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. @MarcoGorelli gentle ping if there are no further comments, i plan to merge this next week sorry for the delay, will try to take a look tomorrow Sorry this has taken a while, just not sure about something

Isn't this going to risk causing overflows? E.g.
```python
ser = pd.Series(np.array(['NaT', '3000-01-01', '3000-01-02'], dtype='datetime64[s]'))

item = pd.Timestamp('2020-01-01T00:00:00.123456789')
print(ser.fillna(item))
```
This raises
```
pandas._libs.tslibs.np_datetime.OutOfBoundsDatetime: Out of bounds nanosecond timestamp: 3000-01-01 00:00:00
```

Checking the behaviour for nullable integers:
```
pd.Series([pd.NA, 2, 3], dtype='Int64').fillna(1.5)  # raises
pd.Series([pd.NA, 2, 3], dtype='Int64[pyarrow]').fillna(1.5)  # silently truncates 1.5 to 1
``` > Isn't this going to risk causing overflows? E.g.

Yes, and that is a good thing.  In the status quo we silently round that non-nano fill value and give an incorrect result.

The pyarrow example looks pretty clearly buggy to me. to be honest I'm not totally sure about this - I'm not going to block it, but don't really feel comfortable approving, could you ask someone else please? @mroeschke @phofl thoughts? I do find the overflow behavior a better behavior than silently truncating.

But I also find the exception message in @MarcoGorelli 's https://github.com/pandas-dev/pandas/pull/56419#issuecomment-2051569888 kinda opaque compared to setting a float into a nullable int (`TypeError: Invalid value '1.5' for dtype Int64`)  It is a bit tough to tailor the exception message due to where the exception is raised.  How about f"Incompatible (high-resolution) value for {self.dtype}. Explicitly cast before operating." > How about f"Incompatible (high-resolution) value for {self.dtype}. Explicitly cast before operating."

Yeah I would be good with that npdev failure looks unrelated Thanks @jbrockmendel 