From f4c65480697d58a4184d564f7d62c90656e7f3a7 Mon Sep 17 00:00:00 2001
From: hauntsaninja <hauntsaninja@gmail.com>
Date: Tue, 12 Aug 2025 15:39:58 -0700
Subject: [PATCH 1/2] Fix inference when unpacking union type

---
 mypy/argmap.py                      | 13 +++++++++++++
 test-data/unit/check-functions.test | 17 +++++++++++++++++
 2 files changed, 30 insertions(+)

diff --git a/mypy/argmap.py b/mypy/argmap.py
index 28fad1f093dd..3bb48fefa9d2 100644
--- a/mypy/argmap.py
+++ b/mypy/argmap.py
@@ -17,6 +17,7 @@
     TypeOfAny,
     TypeVarTupleType,
     UnpackType,
+    UnionType,
     get_proper_type,
 )
 
@@ -211,6 +212,18 @@ def expand_actual_type(
                     # Just return `Any`, other parts of code would raise
                     # a different error for improper use.
                     return AnyType(TypeOfAny.from_error)
+            elif isinstance(actual_type, UnionType):
+                item_types = [
+                    self.expand_actual_type(
+                        item,
+                        actual_kind=actual_kind,
+                        formal_name=formal_name,
+                        formal_kind=formal_kind,
+                        allow_unpack=allow_unpack,
+                    )
+                    for item in actual_type.items
+                ]
+                return UnionType.make_union(item_types)
             elif isinstance(actual_type, TupleType):
                 # Get the next tuple item of a tuple *arg.
                 if self.tuple_index >= len(actual_type.items):
diff --git a/test-data/unit/check-functions.test b/test-data/unit/check-functions.test
index 7fa34a398ea0..683d9c90ee22 100644
--- a/test-data/unit/check-functions.test
+++ b/test-data/unit/check-functions.test
@@ -3708,3 +3708,20 @@ foo(*args)  # E: Argument 1 to "foo" has incompatible type "*list[object]"; expe
 kwargs: dict[str, object]
 foo(**kwargs)  # E: Argument 1 to "foo" has incompatible type "**dict[str, object]"; expected "P"
 [builtins fixtures/dict.pyi]
+
+[case testUnpackUnionStarArgs]
+from __future__ import annotations
+from typing import TypeVar
+T = TypeVar("T")
+
+def f(*args: T) -> T: ...
+
+def star_union_list(x: list[str | None] | list[str]):
+    reveal_type([*x])  # N: Revealed type is "builtins.list[Union[builtins.str, None]]"
+    reveal_type(f(*x))  # N: Revealed type is "Union[builtins.str, None]"
+
+def star_union_list_tuple(x: list[str | None] | tuple[int, int]):
+    reveal_type([*x])  # N: Revealed type is "builtins.list[Union[builtins.str, None, builtins.int]]"
+    reveal_type(f(*x))  # N: Revealed type is "Union[builtins.str, None, builtins.int]"
+
+[builtins fixtures/tuple.pyi]

From 7fd1d1a1d46a665f742d3c0e2910983eff09768f Mon Sep 17 00:00:00 2001
From: "pre-commit-ci[bot]"
 <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Date: Tue, 12 Aug 2025 22:48:04 +0000
Subject: [PATCH 2/2] [pre-commit.ci] auto fixes from pre-commit.com hooks

for more information, see https://pre-commit.ci
---
 mypy/argmap.py | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/mypy/argmap.py b/mypy/argmap.py
index 3bb48fefa9d2..363d76cb7322 100644
--- a/mypy/argmap.py
+++ b/mypy/argmap.py
@@ -16,8 +16,8 @@
     TypedDictType,
     TypeOfAny,
     TypeVarTupleType,
-    UnpackType,
     UnionType,
+    UnpackType,
     get_proper_type,
 )