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Fix crash with tuple unpack inside TypeVar default #20456

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Fix crash with tuple unpack inside TypeVar default #20456

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2 changes: 2 additions & 0 deletions mypy/typeops.py
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Original file line number Diff line number Diff line change
Expand Up @@ -113,6 +113,8 @@ def tuple_fallback(typ: TupleType) -> Instance:
and unpacked_type.type.fullname == "builtins.tuple"
):
items.append(unpacked_type.args[0])
elif isinstance(unpacked_type, TupleType):
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@sterliakov sterliakov Dec 23, 2025

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IIRC the idea was that it should never be true when we call tuple_fallback - we have several places where things like tuple[*tuple[X,Y]] are normalized to a plain tuple, and the original intent was to get rid of such constructs as early as possible. This is fine as a quickfix, but I'm afraid that this will manifest again when such nested tuple is used elsewhere.

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@cdce8p cdce8p Dec 23, 2025

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Not entirely sure about that but isn't the function already doing the flatting? E.g. just the line above flattens builtins.tuple Instances.

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@sterliakov sterliakov Dec 23, 2025

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builtins.tuple is already normalized: you can't rewrite e.g. tuple[str, *tuple[int, ...]] as a single tuple[...] construct. This is only about expanding unpacks of fixed-size TupleType types (which is always possible), see flatten_nested_tuples

items.extend(unpacked_type.items)
else:
raise NotImplementedError
else:
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1 change: 1 addition & 0 deletions test-data/unit/check-typevar-defaults.test
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Expand Up @@ -42,6 +42,7 @@ T4 = TypeVar("T4", int, str, default=int)
T5 = TypeVar("T5", default=S0)
T6 = TypeVar("T6", bound=float, default=S1)
T7 = TypeVar("T7", bound=List[Any], default=List[S0])
T8 = TypeVar("T8", default=Tuple[int, Unpack[Tuple[int, str]]])

P1 = ParamSpec("P1", default=[])
P2 = ParamSpec("P2", default=...)
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