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Fix docs: clarify variance behavior for PEP 695 generics #20971

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Fix docs: clarify variance behavior for PEP 695 generics #20971

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14 changes: 9 additions & 5 deletions docs/source/common_issues.rst
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Expand Up @@ -278,9 +278,7 @@ multiple variables (or maybe declare the variable with an ``Any`` type).
...
n = 'x' # error: Incompatible types in assignment (expression has type "str", variable has type "int")

.. note::

Using the :option:`--allow-redefinition <mypy --allow-redefinition>`
.. note:: Using the :option:`--allow-redefinition <mypy --allow-redefinition>`
flag can suppress this error in several cases.

Note that you can redefine a variable with a more *precise* or a more
Expand All @@ -301,9 +299,15 @@ See :ref:`type-narrowing` for more information.

Invariance vs covariance
------------------------
Most mutable generic collections are invariant. With the older
``TypeVar`` syntax (for example ``T = TypeVar("T")``), user-defined
generic classes are invariant by default.

With the newer PEP 695 syntax (for example ``class MyCls[T]:``),
type parameters may be inferred as covariant if they are not used
in positions that require invariance.


Most mutable generic collections are invariant, and mypy considers all
user-defined generic classes invariant by default
(see :ref:`variance-of-generics` for motivation). This could lead to some
unexpected errors when combined with type inference. For example:

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