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Accept value as keyword argument in TypeAliasType #20556

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Merged
JelleZijlstra merged 1 commit into from
Jan 10, 2026
+16 −2
Merged

Accept value as keyword argument in TypeAliasType #20556

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10 changes: 8 additions & 2 deletions mypy/semanal.py
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Original file line number Diff line number Diff line change
Expand Up @@ -4061,7 +4061,11 @@ def check_and_set_up_type_alias(self, s: AssignmentStmt) -> bool:
type_params: TypeVarLikeList | None
all_type_params_names: list[str] | None
if self.check_type_alias_type_call(s.rvalue, name=lvalue.name):
rvalue = s.rvalue.args[1]
rvalue = (
s.rvalue.args[1]
if s.rvalue.arg_kinds[1] == ARG_POS
else s.rvalue.args[s.rvalue.arg_names.index("value")]
)
pep_695 = True
type_params, all_type_params_names = self.analyze_type_alias_type_params(s.rvalue)
else:
Expand Down Expand Up @@ -4249,7 +4253,9 @@ def check_type_alias_type_call(self, rvalue: Expression, *, name: str) -> TypeGu
return False
if not self.check_typevarlike_name(rvalue, name, rvalue):
return False
if rvalue.arg_kinds.count(ARG_POS) != 2:
if rvalue.arg_kinds.count(ARG_POS) != (
2 - ("value" in rvalue.arg_names) - ("name" in rvalue.arg_names)
):
return False

return True
Expand Down
8 changes: 8 additions & 0 deletions test-data/unit/check-type-aliases.test
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Original file line number Diff line number Diff line change
Expand Up @@ -1071,6 +1071,14 @@ x: TestType = 42
y: TestType = 'a'
z: TestType = object() # E: Incompatible types in assignment (expression has type "object", variable has type "int | str")

TA = TypeAliasType("TA", value=int)
a: TA = 1
b: TA = 's' # E: Incompatible types in assignment (expression has type "str", variable has type "int")

TA2 = TypeAliasType("TA2", type_params=(), value=int) # shuffled arguments
a2: TA2 = 1
b2: TA2 = 's' # E: Incompatible types in assignment (expression has type "str", variable has type "int")

reveal_type(TestType) # N: Revealed type is "typing_extensions.TypeAliasType"
TestType() # E: "TypeAliasType" not callable

Expand Down