Document that nested Annotated types referenced through a type alias are not flattened

Author: vberlier

Doc/library/typing.rst

@@ -1402,6 +1402,14 @@ These can be used as types in annotations. They all support subscription using
             int, ValueRange(3, 10), ctype("char")
         ]
 
+     However, this does not apply to ``Annotated`` types referenced through a type
+     alias::
+
+        type From3To10[T] = Annotated[T, ValueRange(3, 10)]
+        assert Annotated[From3To10[int], ctype("char")] != Annotated[
+            int, ValueRange(3, 10), ctype("char")
+        ]
+
    * Duplicated metadata elements are not removed::
 
         assert Annotated[int, ValueRange(3, 10)] != Annotated[

Lib/typing.py

@@ -2239,6 +2239,12 @@ def Annotated(self, *params):
 
         assert Annotated[Annotated[T, Ann1, Ann2], Ann3] == Annotated[T, Ann1, Ann2, Ann3]
 
+      However, this does not apply to Annotated types referenced through
+      a type alias::
+
+        type A[T] = Annotated[T, Ann1, Ann2]
+        assert Annotated[A[T], Ann3] != Annotated[T, Ann1, Ann2, Ann3]
+
     - Instantiating an annotated type is equivalent to instantiating the
     underlying type::