Improve interaction between --local-partial-types and hashability (python#20719)

Without this, we would reveal `list[Hashable]` in the test case

This isn't a great fix though...

Author: hauntsaninja

mypy/checker.py

@@ -4470,6 +4470,14 @@ def infer_variable_type(
                 is_lvalue_final=name.is_final,
                 is_lvalue_member=isinstance(lvalue, MemberExpr),
             )
+            and not (
+                # Trust None assignments to dunder methods
+                # This is a bit ad-hoc, but it improves protocol
+                # (non-)assignability, for instance `__hash__ = None`
+                self.scope.active_class()
+                and is_dunder(name.name)
+                and isinstance(get_proper_type(init_type), NoneType)
+            )
             and not self.no_partial_types
         ):
             # We cannot use the type of the initialization expression for full type

test-data/unit/check-protocols.test

@@ -2940,7 +2940,25 @@ class Gleemer:
 
 
 [case testPartialTypeProtocolHashable]
-# flags: --no-strict-optional
+# flags: --no-strict-optional --no-local-partial-types
+from typing import Protocol
+
+class Hashable(Protocol):
+    def __hash__(self) -> int: ...
+
+class ObjectHashable:
+    def __hash__(self) -> int: ...
+
+class DataArray(ObjectHashable):
+    __hash__ = None
+
+    def f(self, x: Hashable) -> None:
+        reveal_type([self, x])  # N: Revealed type is "builtins.list[builtins.object]"
+[builtins fixtures/tuple.pyi]
+
+
+[case testPartialTypeProtocolHashableLocalPartialTypes]
+# flags: --no-strict-optional --local-partial-types
 from typing import Protocol
 
 class Hashable(Protocol):