Explain why alias is not flattened

Author: vberlier

Doc/library/typing.rst

@@ -1403,7 +1403,7 @@ These can be used as types in annotations. They all support subscription using
         ]
 
      However, this does not apply to ``Annotated`` types referenced through a type
-     alias::
+     alias, to avoid forcing evaluation of the underlying :class:`TypeAliasType`::
 
         type From3To10[T] = Annotated[T, ValueRange(3, 10)]
         assert Annotated[From3To10[int], ctype("char")] != Annotated[

Lib/typing.py

@@ -2240,7 +2240,8 @@ 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::
+      a type alias, to avoid forcing evaluation of the underlying
+      TypeAliasType::
 
         type A[T] = Annotated[T, Ann1, Ann2]
         assert Annotated[A[T], Ann3] != Annotated[T, Ann1, Ann2, Ann3]