CLN/BUG: fix ndarray assignment may cause unexpected cast
State: closed

- [x] closes #14001
- [x] tests added / passed
- [x] passes `git diff upstream/master | flake8 --diff`
- [x] whatsnew entry

Fixed a bug with assignment against block may be coerced incorrectly.

```
s = pd.Series([pd.Timestamp('2011-01-01'), pd.Timestamp('2012-01-01')])
s[1] = 1
s
#0   2011-01-01 00:00:00.000000000
#1   1970-01-01 00:00:00.000000001
# dtype: datetime64[ns]
```


Labels:
name: Bug
name: Indexing
description: Related to indexing on series/frames, not to indexes themselves
name: Dtype Conversions
description: Unexpected or buggy dtype conversions

Comments:
# [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/14145?src=pr&el=h1) Report
> Merging [#14145](https://codecov.io/gh/pandas-dev/pandas/pull/14145?src=pr&el=desc) into [master](https://codecov.io/gh/pandas-dev/pandas/commit/9ddba8dc811184c49d9bc4df5dc97d505345ec23?src=pr&el=desc) will **increase** coverage by `-1.11%`.


```diff
@@            Coverage Diff             @@
##           master   #14145      +/-   ##
==========================================
- Coverage   86.33%   85.22%   -1.11%     
==========================================
  Files         139      143       +4     
  Lines       51149    50798     -351     
==========================================
- Hits        44157    43295     -862     
- Misses       6992     7503     +511
```


| [Impacted Files](https://codecov.io/gh/pandas-dev/pandas/pull/14145?src=pr&el=tree) | Coverage Δ | |
|---|---|---|
| [pandas/core/panel.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...0fa29115af767d240223a33bb1ecfc64c9332d38?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvcGFuZWwucHk=) | `97.5% <100%> (+0.27%)` | :white_check_mark: |
| [pandas/core/frame.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...0fa29115af767d240223a33bb1ecfc64c9332d38?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvZnJhbWUucHk=) | `97.85% <100%> (-0.02%)` | :x: |
| [pandas/core/internals.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...0fa29115af767d240223a33bb1ecfc64c9332d38?src=pr&el=tree#diff-cGFuZGFzL2NvcmUvaW50ZXJuYWxzLnB5) | `94.38% <96.15%> (+0.2%)` | :white_check_mark: |
| [pandas/types/cast.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...0fa29115af767d240223a33bb1ecfc64c9332d38?src=pr&el=tree#diff-cGFuZGFzL3R5cGVzL2Nhc3QucHk=) | `85.74% <96.29%> (+0.32%)` | :white_check_mark: |
| [pandas/io/html.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...0fa29115af767d240223a33bb1ecfc64c9332d38?src=pr&el=tree#diff-cGFuZGFzL2lvL2h0bWwucHk=) | `64.35% <ø> (-20.14%)` | :x: |
| [pandas/util/clipboard/__init__.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...0fa29115af767d240223a33bb1ecfc64c9332d38?src=pr&el=tree#diff-cGFuZGFzL3V0aWwvY2xpcGJvYXJkL19faW5pdF9fLnB5) | `61.9% <ø> (-11.91%)` | :x: |
| [pandas/io/common.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...0fa29115af767d240223a33bb1ecfc64c9332d38?src=pr&el=tree#diff-cGFuZGFzL2lvL2NvbW1vbi5weQ==) | `62.8% <ø> (-4.59%)` | :x: |
| [pandas/io/sas/sasreader.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...0fa29115af767d240223a33bb1ecfc64c9332d38?src=pr&el=tree#diff-cGFuZGFzL2lvL3Nhcy9zYXNyZWFkZXIucHk=) | `4.34% <ø> (-3.06%)` | :x: |
| [pandas/tools/plotting.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...0fa29115af767d240223a33bb1ecfc64c9332d38?src=pr&el=tree#diff-cGFuZGFzL3Rvb2xzL3Bsb3R0aW5nLnB5) | `68.8% <ø> (-2.99%)` | :x: |
| [pandas/tseries/plotting.py](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...0fa29115af767d240223a33bb1ecfc64c9332d38?src=pr&el=tree#diff-cGFuZGFzL3RzZXJpZXMvcGxvdHRpbmcucHk=) | `62.57% <ø> (-2.17%)` | :x: |
| ... and [69 more](https://codecov.io/gh/pandas-dev/pandas/pull/14145/changes?src=pr&el=tree-more) | |

------

[Continue to review full report at Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/14145?src=pr&el=continue).
> **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta)
> `Δ = absolute <relative> (impact)`, `ø = not affected`, `? = missing data`
> Powered by [Codecov](https://codecov.io/gh/pandas-dev/pandas/pull/14145?src=pr&el=footer). Last update [9ddba8d...14e5f0f](https://codecov.io/gh/pandas-dev/pandas/compare/9ddba8dc811184c49d9bc4df5dc97d505345ec23...14e5f0fd2b665a403baac953f6c7c610ce4324ac?el=footer&src=pr&el=lastupdated). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments).
can you rebase

can you rebase
can you rebase / update
@sinhrks can you rebase / update
have tried but I think this might fix #15743 as well
supeseded by #16408