@hauntsaninja @JelleZijlstra could one of you (or both) please take a look at this one as well?

OK, so mypy_primer is expected, I effectively slightly relaxed rules for --warn-redundant-casts, and slightly enforced them for assert_type(). I used this as a rule of thumb when deciding whether to switch to ==: do I want int and int64 be different under this check? (They are proper subtypes of each other, but have different runtime representation.)

@JukkaL After some thinking, I decided to use ignore_promotions=True for the new implementation of is_same_type(), so e.g. int and int64 will be different and Union[int, bool] (if not simplified) will be different from int. This is somewhat closer to old behavior, although there is no difference in tests and in mypy_primer.

I went again through all call sites (there is just a dozen or so), and I think the semantics introduced by this PR makes sense there.
I remember you was quite cautious about this change few years ago. But with mypy_primer I am more confident now. If you are still against, please let me know, otherwise I will merge this soon.

After working a bit on cleaning get_proper_type() usage, I think I will use is_same_type() in assert_type() (like it was before). Having == there will be a big pain for users if type is recursive alias.

Diff from mypy_primer, showing the effect of this PR on open source code:

spark (https://github.com/apache/spark)
- python/pyspark/sql/pandas/types.py:351: error: Redundant cast to PandasSeriesLike?  [redundant-cast]
- python/pyspark/sql/pandas/types.py:412: error: Redundant cast to PandasSeriesLike?  [redundant-cast]
- python/pyspark/sql/pandas/types.py:422: error: Redundant cast to PandasSeriesLike?  [redundant-cast]

cwltool (https://github.com/common-workflow-language/cwltool)
- cwltool/main.py: note: In function "main":
- cwltool/main.py:1128:26: error: Redundant cast to "Union[Any, Any]"  [redundant-cast]

steam.py (https://github.com/Gobot1234/steam.py)
- steam/package.py:227: error: Unsupported left operand type for & (Self?)  [operator]

core (https://github.com/home-assistant/core)
- homeassistant/components/simplisafe/__init__.py:255: error: Redundant cast to "Union[Any, Any]"  [redundant-cast]