Fix crash on self-referential Generic base (fixes #21549)

class C(Generic[C]) hit "AssertionError: Must not defer during final
iteration" because the placeholder for C never resolves. On the final
iteration, report an error instead of deferring, matching how other
placeholder sites behave. Adds a regression test.

Author: Dextheking1

mypy/semanal.py

@@ -2478,6 +2478,12 @@ def analyze_unbound_tvar_impl(
         assert not is_unpacked or not is_typealias_param, "Mutually exclusive conditions"
         sym = self.lookup_qualified(t.name, t)
         if sym and isinstance(sym.node, PlaceholderNode):
+            if self.final_iteration:
+                # The reference can never be resolved (e.g. a class using itself
+                # in its own bases: `class C(Generic[C])`). Don't defer here, as
+                # that would be an internal error. Fall through so the caller
+                # reports a proper error instead.
+                return None
             self.record_incomplete_ref()
         if not is_unpacked and sym and isinstance(sym.node, ParamSpecExpr):
             if sym.fullname and not self.tvar_scope.allow_binding(sym.fullname):

test-data/unit/semanal-errors.test

@@ -1016,6 +1016,13 @@ class A(Generic[T]):
           # E: Free type variable expected in Generic[...]
 [out]
 
+[case testSelfReferenceInGenericBase]
+from typing import Generic
+class C(Generic[C]): pass \
+    # E: Free type variable expected in Generic[...] \
+    # E: Cannot resolve name "C" (possible cyclic definition)
+[out]
+
 [case testRedeclaredTypeVarWithinNestedGenericClass]
 from typing import Generic, Iterable, TypeVar
 T = TypeVar('T')