Fix inference when unpacking union type

Author: hauntsaninja

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):

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]