PEP 673: forbid Self in type aliases. (#2194)

Thanks to Eric Traut who tested all the code examples in Pyright and
raised implementation concerns about ``Self`` type in aliases.

I ran into implementation issues in Pyre too. This is because Pyre
preprocesses things like ``Self`` in an early phase and does alias
resolution in a later phase. Mixing the two would lead to a lot of
hacks.

pep-0673.rst

@@ -605,11 +605,6 @@ The following uses of ``Self`` are accepted:
         # Accepted (treated as an @property returning the Callable type)
         bar: Callable[[Self], int] = foo
 
-    TupleSelf = Tuple[Self, Self]
-    class Alias:
-        def return_tuple(self) -> TupleSelf:
-            return (self, self)
-
     class HasNestedFunction:
         x: int = 42
 
@@ -660,6 +655,20 @@ The following uses of ``Self`` are rejected.
 
     class Baz(Foo[Self]): ...  # Rejected
 
+We reject type aliases containing ``Self``. Supporting ``Self``
+outside class definitions can require a lot of special-handling in
+type checkers. Given that it also goes against the rest of the PEP to
+use ``Self`` outside a class definition, we believe the added
+convenience of aliases is not worth it:
+
+::
+
+    TupleSelf = Tuple[Self, Self]  # Rejected
+
+    class Alias:
+        def return_tuple(self) -> TupleSelf:  # Rejected
+            return (self, self)
+
 Note that we reject ``Self`` in staticmethods. ``Self`` does not add much
 value since there is no ``self`` or ``cls`` to return. The only possible use
 cases would be to return a parameter itself or some element from a container
@@ -781,8 +790,8 @@ Other languages have similar ways to express the type of the enclosing class:
 
 Thanks to the following people for their feedback on the PEP:
 
-Jia Chen, Rebecca Chen, Sergei Lebedev, Kaylynn Morgan, Tuomas Suutari, Alex
-Waygood, Shannon Zhu, and Никита Соболев
+Jia Chen, Rebecca Chen, Sergei Lebedev, Kaylynn Morgan, Tuomas
+Suutari, Eric Traut, Alex Waygood, Shannon Zhu, and Никита Соболев
 
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