Documents explicit type aliases (#11800)

Refs https://www.python.org/dev/peps/pep-0613/

Co-authored-by: Jelle Zijlstra <jelle.zijlstra@gmail.com>

docs/source/common_issues.rst

@@ -638,39 +638,48 @@ Mypy has both type aliases and variables with types like ``Type[...]`` and it is
 
 1. Variables with type ``Type[...]`` should be created by assignments with an explicit type annotations:
 
-.. code-block:: python
+   .. code-block:: python
 
-    class A: ...
-    tp: Type[A] = A
+     class A: ...
+     tp: Type[A] = A
 
-2. Aliases are created by assignments without an explicit type:
+2. Aliases are created by assignments without an explicit type. 
 
-.. code-block:: python
+   .. code-block:: python
+
+     class A: ...
+     Alias = A
 
-    class A: ...
-    Alias = A
+   Or you can also use :pep:`613` and explicit type aliases:
+
+   .. code-block:: python
+   
+     from typing import TypeAlias  # or `from typing_extensions` before `python3.10`
+     
+     class A: ...
+     Alias: TypeAlias = A
 
 3. The difference is that aliases are completely known statically and can be used in type context (annotations):
 
-.. code-block:: python
+   .. code-block:: python
 
-    class A: ...
-    class B: ...
+     class A: ...
+     class B: ...
 
-    if random() > 0.5:
-        Alias = A
-    else:
-        Alias = B  # error: Cannot assign multiple types to name "Alias" without an explicit "Type[...]" annotation \
-                   # error: Incompatible types in assignment (expression has type "Type[B]", variable has type "Type[A]")
+     if random() > 0.5:
+         Alias = A
+     else:
+         Alias = B  # error: Cannot assign multiple types to name "Alias" without an explicit "Type[...]" annotation \
+                    # error: Incompatible types in assignment (expression has type "Type[B]", variable has type "Type[A]")
 
-    tp: Type[object]  # tp is a type variable
-    if random() > 0.5:
-        tp = A
-    else:
-        tp = B  # This is OK
+     tp: Type[object]  # tp is a type variable
+     if random() > 0.5:
+         tp = A
+     else:
+         tp = B  # This is OK
 
-    def fun1(x: Alias) -> None: ...  # This is OK
-    def fun2(x: tp) -> None: ...  # error: Variable "__main__.tp" is not valid as a type
+     def fun1(x: Alias) -> None: ...  # This is OK
+     def fun2(x: tp) -> None: ...  # error: Variable "__main__.tp" is not valid as a type
 
 Incompatible overrides
 ----------------------

docs/source/kinds_of_types.rst

@@ -362,7 +362,6 @@ is needed:
 
 .. code-block:: python
 
-
    class Container:
        items: list[str]  # No initializer
 
@@ -522,6 +521,21 @@ assigning the type to a variable:
     another type -- it's equivalent to the target type except for
     :ref:`generic aliases <generic-type-aliases>`.
 
+Since Mypy 0.930 you can also use explicit type aliases which are defined by :pep:`613`.
+
+Implicit type alias declaration rules create confusion when type aliases involve forward references,
+invalid types, or violate other restrictions enforced on type alias declaration.
+Because the distinction between an unannotated value and a type alias is implicit,
+ambiguous or incorrect type alias declarations implicitly default to a valid value assignment.
+
+.. code-block:: python
+
+   from typing import TypeAlias  # or `from typing_extensions` before `python3.10`
+
+   AliasType: TypeAlias = Union[list[dict[tuple[int, str], set[int]]], tuple[str, list[str]]]
+
+Explicit type aliases are unambiguous and improve readability.
+
 .. _named-tuples:
 
 Named tuples