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Add changelog for mypy 1.12 #17889

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Initial CHANGELOG for 1.12
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## Next release

## Mypy 1.12

We’ve just uploaded mypy 1.12 to the Python Package Index ([PyPI](https://pypi.org/project/mypy/)). Mypy is a static type
checker for Python. This release includes new features, performance improvements and bug fixes.
You can install it as follows:

python3 -m pip install -U mypy

You can read the full documentation for this release on [Read the Docs](http://mypy.readthedocs.io).

### Support Python 3.12 Syntax for Generics (PEP 695)

Support for the new type parameter syntax introduced in Python 3.12 is now enabled by default,
documented, and no longer experimental. It was available through a feature flag in
mypy 1.11 as an experimental feature.

This example demonstrates the new syntax:

```python
# Generic function
def f[T](x: T) -> T: ...

reveal_type(f(1)) # Revealed type is 'int'

# Generic class
class C[T]:
def __init__(self, x: T) -> None:
self.x = x

c = C('a')
reveal_type(c.x) # Revealed type is 'str'

# Type alias
type A[T] = C[list[T]]
```

For more information, refer to the [documentation](https://mypy.readthedocs.io/en/latest/generics.html).

These improvements are included:

* Document Python 3.12 type parameter syntax (Jukka Lehtosalo, PR [17816](https://github.com/python/mypy/pull/17816))
* Further documentation updates (Jukka Lehtosalo, PR [17826](https://github.com/python/mypy/pull/17826))
* Allow Self return types with contravariance (Jukka Lehtosalo, PR [17786](https://github.com/python/mypy/pull/17786))
* Enable new type parameter syntax by default (Jukka Lehtosalo, PR [17798](https://github.com/python/mypy/pull/17798))
* Generate error if new-style type alias used as base class (Jukka Lehtosalo, PR [17789](https://github.com/python/mypy/pull/17789))
* Inherit variance if base class has explicit variance (Jukka Lehtosalo, PR [17787](https://github.com/python/mypy/pull/17787))
* Fix crash on invalid type var reference (Jukka Lehtosalo, PR [17788](https://github.com/python/mypy/pull/17788))
* Fix covariance of frozen dataclasses (Jukka Lehtosalo, PR [17783](https://github.com/python/mypy/pull/17783))
* Allow covariance with attribute that has "`_`" name prefix (Jukka Lehtosalo, PR [17782](https://github.com/python/mypy/pull/17782))
* Support `Annotated[...]` in new-style type aliases (Jukka Lehtosalo, PR [17777](https://github.com/python/mypy/pull/17777))
* Fix nested generic classes (Jukka Lehtosalo, PR [17776](https://github.com/python/mypy/pull/17776))
* Add detection and error reporting for the use of incorrect expressions within the scope of a type parameter and a type alias (Kirill Podoprigora, PR [17560](https://github.com/python/mypy/pull/17560))

### Basic Support for Python 3.13

This release adds partial support for Python 3.13 features and compiled binaries for
Python 3.13. Mypyc now also supports Python 3.13.

In particular, these features are supported:
* Various new stdlib features and changes (through typeshed stub improvements)
* `typing.ReadOnly` (see below for more)
* `typing.TypeIs` (added in mypy 1.10, [PEP 742](https://peps.python.org/pep-0742/))
* Type parameter defaults when using the legacy syntax ([PEP 696](https://peps.python.org/pep-0696/))

These features are not supported yet:
* `warnings.deprecated` ([PEP 702](https://peps.python.org/pep-0702/))
* Type parameter defaults when using Python 3.12 type parameter syntax

### Mypyc Support for Python 3.13

Mypyc now supports Python 3.13. This was contributed by Marc Mueller, with additional
fixes by Jukka Lehtosalo. Free threaded Python 3.13 builds are not supported yet.

List of changes:

* Add additional includes for Python 3.13 (Marc Mueller, PR [17506](https://github.com/python/mypy/pull/17506))
* Add another include for Python 3.13 (Marc Mueller, PR [17509](https://github.com/python/mypy/pull/17509))
* Fix ManagedDict functions for Python 3.13 (Marc Mueller, PR [17507](https://github.com/python/mypy/pull/17507))
* Update mypyc test output for Python 3.13 (Marc Mueller, PR [17508](https://github.com/python/mypy/pull/17508))
* Fix `PyUnicode` functions for Python 3.13 (Marc Mueller, PR [17504](https://github.com/python/mypy/pull/17504))
* Fix `_PyObject_LookupAttrId` for Python 3.13 (Marc Mueller, PR [17505](https://github.com/python/mypy/pull/17505))
* Fix `_PyList_Extend` for Python 3.13 (Marc Mueller, PR [17503](https://github.com/python/mypy/pull/17503))
* Fix `gen_is_coroutine` for Python 3.13 (Marc Mueller, PR [17501](https://github.com/python/mypy/pull/17501))
* Fix `_PyObject_FastCall` for Python 3.13 (Marc Mueller, PR [17502](https://github.com/python/mypy/pull/17502))
* Avoid uses of `_PyObject_CallMethodOneArg` on 3.13 (Jukka Lehtosalo, PR [17526](https://github.com/python/mypy/pull/17526))
* Don't rely on `_PyType_CalculateMetaclass` on 3.13 (Jukka Lehtosalo, PR [17525](https://github.com/python/mypy/pull/17525))
* Don't use `_PyUnicode_FastCopyCharacters` on 3.13 (Jukka Lehtosalo, PR [17524](https://github.com/python/mypy/pull/17524))
* Don't use `_PyUnicode_EQ` on 3.13, as it's no longer exported (Jukka Lehtosalo, PR [17523](https://github.com/python/mypy/pull/17523))

### Inferring Unions for Conditional Expressions

Mypy now always tries to infer a union type for a conditional expression if left and right
operand types are different. This results in more precise inferred types and lets mypy detect
more issues. Example:

```python
s = "foo" if cond() else 1
# Type of "s" is now "str | int" (it used to be "object")
```

Notably, if one of the operands has type `Any`, the type of a conditional expression is
now `<type> | Any`. Previously the inferred type was just `Any`. The new type essentially
indicates that the value can be of type `<type>`, and potentially of some (unknown) type.
Most operations performed on the result must also be valid for `<type>`.
Example where this is relevant:

```python
from typing import Any

def func(a: Any, b: bool) -> None:
x = a if b else None
# Type of x is "Any | None"
print(x.y) # Error: None has no attribute "y"
```

This feature was contributed by Ivan Levkivskyi (PR [17427](https://github.com/python/mypy/pull/17427)).

### ReadOnly Support for TypedDict (PEP 705)

You can now use `typing.ReadOnly` to specity TypedDict items as
read-only ([PEP 705](https://peps.python.org/pep-0705/)):

```python
from typing import TypedDict

# Or "from typing ..." on Python 3.13
from typing_extensions import ReadOnly

class TD(TypedDict):
a: int
b: ReadOnly[int]

d: TD = {"a": 1, "b": 2}
d["a"] = 3 # OK
d["b"] = 5 # Error: "b" is ReadOnly
```

This feature was contributed by Nikita Sobolev (PR [17644](https://github.com/python/mypy/pull/17644)).

### Python 3.8 End of Life Approaching

We are planning to drop support for Python 3.8 in the next mypy feature release or the
one after that. Python 3.8 reaches end of life in October 2024.

### Planned Changes to Defaults

We are planning to enable `--local-partial-types` by default in mypy 2.0. This will
often require at least minor code changes. This option is implicitly enabled by mypy
daemon, so this makes the behavior of daemon and non-daemon modes consistent.

We recommend that mypy users start using local partial types soon (or to explicitly disable
them) to prepare for the change.

This can also be configured in a mypy configuration file:

```
local_partial_types = True
```

For more information, refer to the
[documentation](https://mypy.readthedocs.io/en/stable/command_line.html#cmdoption-mypy-local-partial-types).

### Documentation Updates

Mypy documentation now uses modern syntax variants and imports in many examples. Some
examples no longer work on Python 3.8, which is the earliest Python version that mypy supports.

Notably, `Iterable` and other protocols/ABCs are imported from `collections.abc` instead of
`typing`:
```python
from collections.abc import Iterable, Callable
```

Examples also avoid the upper-case aliases to built-in types: `list[str]` is used instead
of `List[str]`. The `X | Y` union type syntax introduced in Python 3.10 is also now prevalent.

List of documentation updates:

* Document `--output=json` CLI option (Edgar Ramírez Mondragón, PR [17611](https://github.com/python/mypy/pull/17611))
* Update various references to deprecated type aliases in docs (Jukka Lehtosalo, PR [17829](https://github.com/python/mypy/pull/17829))
* Make "X | Y" union syntax more prominent in documentation (Jukka Lehtosalo, PR [17835](https://github.com/python/mypy/pull/17835))
* Discuss upper bounds before self types in documentation (Jukka Lehtosalo, PR [17827](https://github.com/python/mypy/pull/17827))
* Make changelog visible in mypy documentation (quinn-sasha, PR [17742](https://github.com/python/mypy/pull/17742))
* List all incomplete features in `--enable-incomplete-feature` docs (sobolevn, PR [17633](https://github.com/python/mypy/pull/17633))
* Remove the explicit setting of a pygments theme (Pradyun Gedam, PR [17571](https://github.com/python/mypy/pull/17571))

### Experimental Inline TypedDict Syntax

Mypy now supports a non-standard, experimental syntax for defining anonymous TypedDicts.
Example:

```python
def func(n: str, y: int) -> {"name": str, "year": int}:
return {"name": n, "year": y}
```

The feature is disabled by default. Use `--enable-incomplete-feature=InlineTypedDict` to
enable it. *We might remove this feature in a future release.*

This feature was contributed by Ivan Levkivskyi (PR [17457](https://github.com/python/mypy/pull/17457)).

### Stubgen Improvements

* Fix crash on literal class-level keywords (sobolevn, PR [17663](https://github.com/python/mypy/pull/17663))
* Stubgen add `--version` (sobolevn, PR [17662](https://github.com/python/mypy/pull/17662))
* Fix `stubgen --no-analysis/--parse-only` docs (sobolevn, PR [17632](https://github.com/python/mypy/pull/17632))
* Include keyword only args when generating signatures in stubgenc (Eric Mark Martin, PR [17448](https://github.com/python/mypy/pull/17448))
* Add Literal support for docstrings (Michael Carlstrom, PR [17441](https://github.com/python/mypy/pull/17441))
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* Use `Generator` type var defaults (Sebastian Rittau, PR [17670](https://github.com/python/mypy/pull/17670))

### Stubtest Improvements
* Add support for `cached_property` (Ali Hamdan, PR [17626](https://github.com/python/mypy/pull/17626))
* Add `enable_incomplete_feature` validation to `stubtest` (sobolevn, PR [17635](https://github.com/python/mypy/pull/17635))
* Fix error code handling in `stubtest` with `--mypy-config-file` (sobolevn, PR [17629](https://github.com/python/mypy/pull/17629))

### Other Notables Fixes and Improvements

* Report error if using unsupported type parameter defaults (Jukka Lehtosalo, PR [17876](https://github.com/python/mypy/pull/17876))
* Fix re-processing cross-reference in mypy daemon when node kind changes (Ivan Levkivskyi, PR [17883](https://github.com/python/mypy/pull/17883))
* Don't use equality to narrow when value is IntEnum/StrEnum (Jukka Lehtosalo, PR [17866](https://github.com/python/mypy/pull/17866))
* Don't consider None vs IntEnum comparison ambiguous (Jukka Lehtosalo, PR [17877](https://github.com/python/mypy/pull/17877))
* Fix narrowing of IntEnum and StrEnum types (Jukka Lehtosalo, PR [17874](https://github.com/python/mypy/pull/17874))
* Filter overload items based on self type during type inference (Jukka Lehtosalo, PR [17873](https://github.com/python/mypy/pull/17873))
* Enable negative narrowing of union TypeVar upper bounds (Brian Schubert, PR [17850](https://github.com/python/mypy/pull/17850))
* Fix get_member_expr_fullname returning strings with embedded "None" (Brian Schubert, PR [17848](https://github.com/python/mypy/pull/17848))
* Avoid type size explosion when expanding types (Jukka Lehtosalo, PR [17842](https://github.com/python/mypy/pull/17842))
* Fix negative narrowing of tuples in match statement (Brian Schubert, PR [17817](https://github.com/python/mypy/pull/17817))
* Narrow falsey str/bytes/int to literal type (Brian Schubert, PR [17818](https://github.com/python/mypy/pull/17818))
* Test against latest Python 3.13, make testing 3.14 easy (Shantanu, PR [17812](https://github.com/python/mypy/pull/17812))
* Reject ParamSpec-typed callables calls with insufficient arguments (Stanislav Terliakov, PR [17323](https://github.com/python/mypy/pull/17323))
* Fix crash when passing too many type arguments to generic base class accepting single ParamSpec (Brian Schubert, PR [17770](https://github.com/python/mypy/pull/17770))
* Fix TypeVar upper bounds sometimes not being displayed in pretty callables (Brian Schubert, PR [17802](https://github.com/python/mypy/pull/17802))
* Added error code for overlapping function signatures (Katrina Connors, PR [17597](https://github.com/python/mypy/pull/17597))
* Check for `truthy-bool` in `not ...` unary expressions (sobolevn, PR [17773](https://github.com/python/mypy/pull/17773))
* Add missing lines-covered and lines-valid attributes (Soubhik Kumar Mitra, PR [17738](https://github.com/python/mypy/pull/17738))
* Fix another crash scenario on recursive tuple types (Ivan Levkivskyi, PR [17708](https://github.com/python/mypy/pull/17708))
* Resolve TypeVar upper bounds in `functools.partial` (Shantanu, PR [17660](https://github.com/python/mypy/pull/17660))
* Always reset binder when checking deferred nodes (Ivan Levkivskyi, PR [17643](https://github.com/python/mypy/pull/17643))
* Fix crash on a callable attribute with single unpack (Ivan Levkivskyi, PR [17641](https://github.com/python/mypy/pull/17641))
* Fix mismatched signature between checker plugin API and implementation (bzoracler, PR [17343](https://github.com/python/mypy/pull/17343))
* Indexing a type also produces a GenericAlias (Shantanu, PR [17546](https://github.com/python/mypy/pull/17546))
* Fix crash on self-type in callable protocol (Ivan Levkivskyi, PR [17499](https://github.com/python/mypy/pull/17499))
* Fix crash on NamedTuple with method and error in function (Ivan Levkivskyi, PR [17498](https://github.com/python/mypy/pull/17498))
* Add `__replace__` for dataclasses in 3.13 (Max Muoto, PR [17469](https://github.com/python/mypy/pull/17469))
* Fix help message for `--no-namespace-packages` (Raphael Krupinski, PR [17472](https://github.com/python/mypy/pull/17472))
* Fix typechecking for async generators (Danny Yang, PR [17452](https://github.com/python/mypy/pull/17452))
* Fix strict optional handling in attrs plugin (Ivan Levkivskyi, PR [17451](https://github.com/python/mypy/pull/17451))
* Allow mixing ParamSpec and TypeVarTuple in Generic (Ivan Levkivskyi, PR [17450](https://github.com/python/mypy/pull/17450))

### Typeshed Updates

Please see [git log](https://github.com/python/typeshed/commits/main?after=91a58b07cdd807b1d965e04ba85af2adab8bf924+0&branch=main&path=stdlib) for full list of standard library typeshed stub changes.

### Acknowledgements
Thanks to all mypy contributors who contributed to this release:

- TODO
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I’d also like to thank my employer, Dropbox, for supporting mypy development.


## Mypy 1.11

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