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Fix crash on union replacement when expanding a variadic tuple unpack #22109

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Fix crash on union replacement when expanding a variadic tuple unpack #22109

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be27ae9
Fix crash on union replacement when expanding a variadic tuple unpack
Madin-H23 Oct 5, 2026
8aacec2
Trigger CI re-run: previous run lost runner communication (infra fail…
Madin-H23 Oct 5, 2026
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4 changes: 3 additions & 1 deletion mypy/expandtype.py
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Original file line number Diff line number Diff line change
Expand Up @@ -392,9 +392,11 @@ def expand_unpack(self, t: UnpackType) -> list[Type]:
or isinstance(repl, TypeVarTupleType)
):
return [UnpackType(typ=repl)]
elif isinstance(repl, (AnyType, UninhabitedType)):
elif isinstance(repl, (AnyType, UninhabitedType, UnionType)):
# Replace *Ts = Any with *Ts = *tuple[Any, ...] and same for Never.
# These types may appear here as a result of user error or failed inference.
# Unions may be inferred from e.g. conditional expressions used as type
# arguments (GH#22108) — carry them over instead of crashing.
return [UnpackType(t.type.tuple_fallback.copy_modified(args=[repl]))]
else:
raise RuntimeError(f"Invalid type replacement to expand: {repl}")
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9 changes: 9 additions & 0 deletions mypy/test/testtypes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -297,6 +297,15 @@ def test_expand_naked_type_var(self) -> None:
self.assert_expand(self.fx.t, [(self.fx.t.id, self.fx.a)], self.fx.a)
self.assert_expand(self.fx.t, [(self.fx.s.id, self.fx.a)], self.fx.t)

def test_expand_unpack_union_replacement(self) -> None:
# GH#22108: expanding *Ts with a union replacement (e.g. inferred from a
# conditional expression) must not crash expand_unpack — the union is
# carried over into the expansion, same as the Any/Never replacements.
union_repl = UnionType([self.tuple(UnpackType(self.tuple(self.fx.anyt))), self.fx.anyt])
orig = self.tuple(UnpackType(self.fx.ts))
exp = mypy.expandtype.expand_type(orig, {self.fx.ts.id: union_repl})
self.assertIn("Any", str(exp))

def test_expand_basic_generic_types(self) -> None:
self.assert_expand(self.fx.gt, [(self.fx.t.id, self.fx.a)], self.fx.ga)

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