Fix crash on self-referential Generic base - #22046
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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.
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Fixes #21549.
class C(Generic[C])crashed with "AssertionError: Must not defer during final iteration". The placeholder forCcan never resolve (the class is still being defined), so the deferral inanalyze_unbound_tvar_impllooped until the final iteration and hit the assert.Changed
analyze_unbound_tvar_implinmypy/semanal.pyto not defer on the final iteration when the looked-up symbol is still a placeholder. It falls through and the caller reports "Free type variable expected in Generic[...]" plus "Cannot resolve name" instead of crashing. This matches the existing convention thatdefer()must never run on the final iteration.Verified:
class C(Generic[C])andclass E(Generic["E"])no longer crash, both now report errors.class D(Generic[T])withTdefined later) still resolve, no new errors.testSelfReferenceInGenericBaseintest-data/unit/semanal-errors.test.mypy/test/testsemanal.py: 578 passed.mypy/test/testcheck.py(generics, classes, protocols, typevar-tuple suites): 1159 passed, 3 skipped, 1 xfailed.mypy/semanal.py: clean.