Fix crash on overriding with frozen attrs (#14186)

Fixes #6715 

Fix is straightforward, currently we assume that if we have a variable
in MRO, and its name appears in current class, it is from this class,
which in fact may not be the case when a variable is overridden with a
property or method.

I also add a test case for a crash that was previously reported in the
same issue but is already (accidentally?) fixed.

mypy/plugins/attrs.py

@@ -736,7 +736,11 @@ def _make_frozen(ctx: mypy.plugin.ClassDefContext, attributes: list[Attribute])
         if attribute.name in ctx.cls.info.names:
             # This variable belongs to this class so we can modify it.
             node = ctx.cls.info.names[attribute.name].node
-            assert isinstance(node, Var)
+            if not isinstance(node, Var):
+                # The superclass attribute was overridden with a non-variable.
+                # No need to do anything here, override will be verified during
+                # type checking.
+                continue
             node.is_property = True
         else:
             # This variable belongs to a super class so create new Var so we

test-data/unit/check-attr.test

@@ -1788,3 +1788,49 @@ class C:
 c = C(x=[C.D()])
 reveal_type(c.x)  # N: Revealed type is "builtins.list[__main__.C.D]"
 [builtins fixtures/list.pyi]
+
+[case testRedefinitionInFrozenClassNoCrash]
+import attr
+
+@attr.s
+class MyData:
+    is_foo: bool = attr.ib()
+
+    @staticmethod  # E: Name "is_foo" already defined on line 5
+    def is_foo(string: str) -> bool: ...
+[builtins fixtures/classmethod.pyi]
+
+[case testOverrideWithPropertyInFrozenClassNoCrash]
+from attrs import frozen
+
+@frozen(kw_only=True)
+class Base:
+    name: str
+
+@frozen(kw_only=True)
+class Sub(Base):
+    first_name: str
+    last_name: str
+
+    @property
+    def name(self) -> str: ...
+[builtins fixtures/property.pyi]
+
+[case testOverrideWithPropertyInFrozenClassChecked]
+from attrs import frozen
+
+@frozen(kw_only=True)
+class Base:
+    name: str
+
+@frozen(kw_only=True)
+class Sub(Base):
+    first_name: str
+    last_name: str
+
+    @property
+    def name(self) -> int: ...  # E: Signature of "name" incompatible with supertype "Base"
+
+# This matches runtime semantics
+reveal_type(Sub)  # N: Revealed type is "def (*, name: builtins.str, first_name: builtins.str, last_name: builtins.str) -> __main__.Sub"
+[builtins fixtures/property.pyi]