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Fix swapping TypeVars with defaults. #19448

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Fix swapping TypeVars with defaults. #19448

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12 changes: 8 additions & 4 deletions mypy/expandtype.py
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Original file line number Diff line number Diff line change
Expand Up @@ -246,10 +246,14 @@ def visit_type_var(self, t: TypeVarType) -> Type:
if (tvar_id := repl.id) in self.recursive_tvar_guard:
return self.recursive_tvar_guard[tvar_id] or repl
self.recursive_tvar_guard[tvar_id] = None
repl = repl.accept(self)
if isinstance(repl, TypeVarType):
repl.default = repl.default.accept(self)
self.recursive_tvar_guard[tvar_id] = repl
expanded = repl.accept(self)

if isinstance(expanded, TypeVarType):
expanded.default = expanded.default.accept(self)
else:
repl = expanded
Comment on lines +251 to +254
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@randolf-scholz randolf-scholz Jul 15, 2025

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maybe more readable if done the other way round?

expanded = repl.accept(self)
if not isinstance(expanded, TypeVarType):
    repl = expanded
else:
    expanded.default = expanded.default.accept(self)


self.recursive_tvar_guard[tvar_id] = expanded
return repl

def visit_param_spec(self, t: ParamSpecType) -> Type:
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18 changes: 18 additions & 0 deletions test-data/unit/check-typevar-defaults.test
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Expand Up @@ -416,6 +416,24 @@ def func_c4(
reveal_type(m) # N: Revealed type is "__main__.ClassC4[builtins.int, builtins.float]"
[builtins fixtures/tuple.pyi]

[case testTypeVarDefaultsSwap]
from typing import TypeVar, Generic

T = TypeVar("T")
X = TypeVar("X", default=object)
Y = TypeVar("Y", default=object)


class Foo(Generic[T, Y]):
def test(self) -> None:
reveal_type( Foo[Y, T]() ) # N: Revealed type is "__main__.Foo[Y`2 = builtins.object, T`1]"


class Bar(Generic[X, Y]):
def test(self) -> None:
reveal_type( Bar[Y, X]() ) # N: Revealed type is "__main__.Bar[Y`2 = builtins.object, Y`2 = builtins.object]"


[case testTypeVarDefaultsClassRecursive1]
# flags: --disallow-any-generics
from typing import Generic, TypeVar, List
Expand Down