Always freeze type variables that were previously freshen (#8075)

Fixes #8072

The fix is straightforward, just copy the logic from the working case (see last test) to two other code paths. Note that only two first cases I added crash on master, but I also add the last one for completeness because I didn't find a similar existing test case.

mypy/checkmember.py

@@ -569,9 +569,10 @@ def analyze_var(name: str,
                 dispatched_type = meet.meet_types(mx.original_type, itype)
                 signature = freshen_function_type_vars(functype)
                 signature = check_self_arg(signature, dispatched_type, var.is_classmethod,
-                                          mx.context, name, mx.msg)
+                                           mx.context, name, mx.msg)
                 signature = bind_self(signature, mx.self_type, var.is_classmethod)
                 expanded_signature = get_proper_type(expand_type_by_instance(signature, itype))
+                freeze_type_vars(expanded_signature)
                 if var.is_property:
                     # A property cannot have an overloaded type => the cast is fine.
                     assert isinstance(expanded_signature, CallableType)
@@ -854,6 +855,7 @@ class B(A[str]): pass
             t = bind_self(t, original_type, is_classmethod=True)
             assert isuper is not None
             t = cast(CallableType, expand_type_by_instance(t, isuper))
+            freeze_type_vars(t)
         return t.copy_modified(variables=tvars + t.variables)
     elif isinstance(t, Overloaded):
         return Overloaded([cast(CallableType, add_class_tvars(item, isuper,

test-data/unit/check-serialize.test

@@ -1336,3 +1336,61 @@ import b
 def foo() -> None:
     class Foo:
         class Bar: pass
+
+[case testSerializeGenericClassMethod]
+import a
+[file a.py]
+import b
+from typing import TypeVar
+
+T = TypeVar('T')
+
+class C:
+    @classmethod
+    def f(cls, x: T) -> T: ...
+
+x = C.f
+[file b.py]
+x = 1
+[file b.py.2]
+x = 'yes'
+[builtins fixtures/classmethod.pyi]
+
+[case testSerializeGenericAbstractMethod]
+import a
+[file a.py]
+import b
+from typing import TypeVar
+from abc import abstractmethod
+
+T = TypeVar('T')
+
+class C:
+    @abstractmethod
+    def f(self, x: T) -> T: ...
+
+c: C
+x = c.f
+[file b.py]
+x = 1
+[file b.py.2]
+x = 'yes'
+
+[case testSerializeGenericNormalMethod]
+import a
+[file a.py]
+import b
+from typing import TypeVar
+from abc import abstractmethod
+
+T = TypeVar('T')
+
+class C:
+    def f(self, x: T) -> T: ...
+
+c: C
+x = c.f
+[file b.py]
+x = 1
+[file b.py.2]
+x = 'yes'