Prevent a crash when InitVar is redefined with a method in a subclass (#19453)

Fixes #19443. This case is too niche (and should be trivially
avoidable), so just not crashing should be good enough. The value is
indeed redefined, and trying to massage the plugin to move the
`X-redefinition` back to `X` in names is not worth the effort IMO.

Author: sterliakov

mypy/checker.py

@@ -2169,7 +2169,14 @@ def check_method_override_for_base_with_name(
         else:
             override_class_or_static = defn.func.is_class or defn.func.is_static
         typ, _ = self.node_type_from_base(defn.name, defn.info, defn)
-        assert typ is not None
+        if typ is None:
+            # This may only happen if we're checking `x-redefinition` member
+            # and `x` itself is for some reason gone. Normally the node should
+            # be reachable from the containing class by its name.
+            # The redefinition is never removed, use this as a sanity check to verify
+            # the reasoning above.
+            assert f"{defn.name}-redefinition" in defn.info.names
+            return False
 
         original_node = base_attr.node
         # `original_type` can be partial if (e.g.) it is originally an

test-data/unit/check-dataclasses.test

@@ -2666,3 +2666,19 @@ class PersonBad(TypedDict):
 class JobBad:
     person: PersonBad = field(default_factory=PersonBad)  # E: Argument "default_factory" to "field" has incompatible type "type[PersonBad]"; expected "Callable[[], PersonBad]"
 [builtins fixtures/dict.pyi]
+
+[case testDataclassInitVarRedefinitionNoCrash]
+# https://github.com/python/mypy/issues/19443
+from dataclasses import InitVar, dataclass
+
+class ClassA:
+    def value(self) -> int:
+        return 0
+
+@dataclass
+class ClassB(ClassA):
+    value: InitVar[int]
+
+    def value(self) -> int:  # E: Name "value" already defined on line 10
+        return 0
+[builtins fixtures/dict.pyi]