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## Summary
When entering an `infer_expression_types` cycle from
`TypeInferenceBuilder::infer_standalone_expression`, we might get back a
`TypeInference::cycle_fallback(…)` that doesn't actually contain any new
types, but instead it contains a `cycle_fallback_type` which is set to
`Some(Type::Never)`. When calling `self.extend(…)`, we therefore don't
really pull in a type for the expression we're interested in. This
caused us to panic if we tried to call `self.expression_type(…)` after
`self.extend(…)`.
The proposed fix here is to retrieve that type from the nested
`TypeInferenceBuilder` directly, which will correctly fall back to
`cycle_fallback_type`.
## Details
I minimized the second example from #17792 a bit further and used this
example for debugging:
```py
from __future__ import annotations
class C: ...
def f(arg: C):
pass
x, _ = f(1)
assert x
```
This is self-referential because when we check the assignment statement
`x, _ = f(1)`, we need to look up the signature of `f`. Since evaluation
of annotations is deferred, we look up the public type of `C` for the
`arg` parameter. The public use of `C` is visibility-constraint by "`x`"
via the `assert` statement. While evaluating this constraint, we need to
look up the type of `x`, which in turn leads us back to the `x, _ =
f(1)` definition.
The reason why this only showed up in the relatively peculiar case with
unpack assignments is the code here:
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| mdtest.py | ||
| mdtest.py.lock | ||