Do not push partial types to the binder (#20202)

Fixes #19996.

This was unearthed by #19400 where `is_subtype(Partial<X>, Partial<X>)`
started returning True - before that `binder.assign_type(expr,
Partial<None>, Partial<None>)` just returned early. If I understand
correctly, that is how Partial should be handled here: we do not want to
push partials to the binder. I do not think we should add a special case
for that (both False and True make some sense for a partial type, I am
not convinced that either one is marginally better), so I just add an
explicit guard to skip adding partial types here.

Author: sterliakov

mypy/checker.py

@@ -3424,7 +3424,11 @@ def check_assignment(
                             and lvalue_type is not None
                         ):
                             lvalue.node.type = remove_instance_last_known_values(lvalue_type)
-                elif self.options.allow_redefinition_new and lvalue_type is not None:
+                elif (
+                    self.options.allow_redefinition_new
+                    and lvalue_type is not None
+                    and not isinstance(lvalue_type, PartialType)
+                ):
                     # TODO: Can we use put() here?
                     self.binder.assign_type(lvalue, lvalue_type, lvalue_type)
 

test-data/unit/check-redefine2.test

@@ -369,6 +369,16 @@ class C4:
         reveal_type(self.x) # N: Revealed type is "builtins.list[builtins.str]"
 
 reveal_type(C4().x) # N: Revealed type is "builtins.list[builtins.str]"
+
+class C5:
+    def __init__(self) -> None:
+        if int():
+            self.x = None
+            return
+        self.x = [""]
+        reveal_type(self.x) # N: Revealed type is "builtins.list[builtins.str]"
+
+reveal_type(C5().x) # N: Revealed type is "Union[builtins.list[builtins.str], None]"
 [builtins fixtures/list.pyi]
 
 [case testNewRedefinePartialGenericTypes]