stubtest: basic support for unpack kwargs

Fixes #21023

Author: hauntsaninja

mypy/stubtest.py

@@ -941,7 +941,24 @@ def from_funcitem(stub: nodes.FuncItem) -> Signature[nodes.Argument]:
             elif stub_arg.kind == nodes.ARG_STAR:
                 stub_sig.varpos = stub_arg
             elif stub_arg.kind == nodes.ARG_STAR2:
-                stub_sig.varkw = stub_arg
+                if stub_arg.variable.type is not None and isinstance(
+                    (typed_dict_arg := mypy.types.get_proper_type(stub_arg.variable.type)),
+                    mypy.types.TypedDictType,
+                ):
+                    for key_name, key_type in typed_dict_arg.items.items():
+                        stub_sig.kwonly[key_name] = nodes.Argument(
+                            nodes.Var(key_name, key_type),
+                            type_annotation=key_type,
+                            initializer=(
+                                nodes.EllipsisExpr()
+                                if key_name in typed_dict_arg.required_keys
+                                else None
+                            ),
+                            kind=nodes.ARG_NAMED,
+                            pos_only=False,
+                        )
+                else:
+                    stub_sig.varkw = stub_arg
             else:
                 raise AssertionError
         return stub_sig

mypy/test/teststubtest.py

@@ -60,6 +60,7 @@ def __getitem__(self, typeargs: Any) -> object: ...
 Final = 0
 Literal = 0
 TypedDict = 0
+Unpack = 0
 
 class TypeVar:
     def __init__(self, name, covariant: bool = ..., contravariant: bool = ...) -> None: ...
@@ -765,6 +766,27 @@ def test_varargs_varkwargs(self) -> Iterator[Case]:
             error="k6",
         )
 
+    @collect_cases
+    def test_kwargs_unpack_typeddict(self) -> Iterator[Case]:
+        yield Case(
+            stub="""
+            from typing import TypedDict, Unpack
+
+            class _Args(TypedDict):
+                a: int
+                b: int
+
+            def f1(**kwargs: Unpack[_Args]) -> None: ...
+            """,
+            runtime="def f1(*, a, b): pass",
+            error=None,
+        )
+        yield Case(
+            stub="def f2(**kwargs: Unpack[_Args]) -> None: ...",
+            runtime="def f2(*, a, c): pass",
+            error="f2",
+        )
+
     @collect_cases
     def test_overload(self) -> Iterator[Case]:
         yield Case(