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Fix noncommutative joins with bounded TypeVars #20345

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Fix noncommutative joins with bounded TypeVars #20345

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47ebf8d
Fix noncommutative joins with bounded TypeVars
hauntsaninja Dec 1, 2025
6c6fa8f
test
hauntsaninja Dec 2, 2025
43a70bb
[pre-commit.ci] auto fixes from pre-commit.com hooks
pre-commit-ci[bot] Dec 2, 2025
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13 changes: 9 additions & 4 deletions mypy/join.py
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Original file line number Diff line number Diff line change
Expand Up @@ -297,10 +297,15 @@ def visit_erased_type(self, t: ErasedType) -> ProperType:
return self.s

def visit_type_var(self, t: TypeVarType) -> ProperType:
if isinstance(self.s, TypeVarType) and self.s.id == t.id:
if self.s.upper_bound == t.upper_bound:
return self.s
return self.s.copy_modified(upper_bound=join_types(self.s.upper_bound, t.upper_bound))
if isinstance(self.s, TypeVarType):
if self.s.id == t.id:
if self.s.upper_bound == t.upper_bound:
return self.s
return self.s.copy_modified(
upper_bound=join_types(self.s.upper_bound, t.upper_bound)
)
# Fix non-commutative joins
return get_proper_type(join_types(self.s.upper_bound, t.upper_bound))
else:
return self.default(self.s)

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29 changes: 29 additions & 0 deletions mypy/test/testtypes.py
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Original file line number Diff line number Diff line change
Expand Up @@ -1051,6 +1051,35 @@ def test_join_type_type_type_var(self) -> None:
self.assert_join(self.fx.type_a, self.fx.t, self.fx.o)
self.assert_join(self.fx.t, self.fx.type_a, self.fx.o)

def test_join_type_var_bounds(self) -> None:
tvar1 = TypeVarType(
"tvar1",
"tvar1",
TypeVarId(-100),
[],
self.fx.o,
AnyType(TypeOfAny.from_omitted_generics),
INVARIANT,
)
any_type = AnyType(TypeOfAny.special_form)
tvar2 = TypeVarType(
"tvar2",
"tvar2",
TypeVarId(-101),
[],
upper_bound=UnionType(
[
TupleType([any_type], self.fx.std_tuple),
TupleType([any_type, any_type], self.fx.std_tuple),
]
),
default=AnyType(TypeOfAny.from_omitted_generics),
variance=INVARIANT,
)

self.assert_join(tvar1, tvar2, self.fx.o)
self.assert_join(tvar2, tvar1, self.fx.o)

# There are additional test cases in check-inference.test.

# TODO: Function types + varargs and default args.
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