Implement closed support in expandtype

Close the kwargs TypedDict returned by ParamSpec where possible. Use
an empty, closed TypedDict instead of a `dict[str, Never]` for
functions that do not accept kwargs.

Author: alicederyn

mypy/expandtype.py

@@ -344,7 +344,7 @@ def _possible_callable_kwargs(cls, repl: Parameters, dict_type: Instance) -> Pro
             return dict_type
         kwargs = {}
         required_names = set()
-        extra_items: Type = UninhabitedType()
+        extra_items: Type | None = None
         for kind, name, type in zip(repl.arg_kinds, repl.arg_names, repl.arg_types):
             if kind == ArgKind.ARG_NAMED and name is not None:
                 kwargs[name] = type
@@ -354,11 +354,11 @@ def _possible_callable_kwargs(cls, repl: Parameters, dict_type: Instance) -> Pro
                 extra_items = type
             elif not kind.is_star() and name is not None:
                 kwargs[name] = type
-        if not kwargs:
+        if not kwargs and extra_items is not None:
             return Instance(dict_type.type, [dict_type.args[0], extra_items])
-        # TODO: when PEP 728 is implemented, pass extra_items below.
-        # TODO: implement closure analysis
-        return TypedDictType(kwargs, required_names, set(), False, fallback=dict_type)
+        # TODO: when PEP 728 `extra_items` is implemented, pass extra_items below.
+        is_closed = extra_items is None
+        return TypedDictType(kwargs, required_names, set(), is_closed, fallback=dict_type)
 
     def visit_type_var_tuple(self, t: TypeVarTupleType) -> Type:
         # Sometimes solver may need to expand a type variable with (a copy of) itself

test-data/unit/check-parameter-specification.test

@@ -2742,16 +2742,16 @@ def f8(x: int, /, **kwargs: str) -> int:
 def f9(x: int, **kwargs: Unpack[Opt]) -> int:
     return 0
 
-reveal_type(Sneaky(f1).kwargs)  # N: Revealed type is "builtins.dict[builtins.str, Never]"
-reveal_type(Sneaky(f2, 1).kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int})"
-reveal_type(Sneaky(f3, 1).kwargs)  # N: Revealed type is "builtins.dict[builtins.str, Never]"
-reveal_type(Sneaky(f4, x=1).kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x': builtins.int})"
-reveal_type(Sneaky(f5, 1).kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int, 'y'?: builtins.int})"
-reveal_type(Sneaky(f5, 1, 2).kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int, 'y'?: builtins.int})"
+reveal_type(Sneaky(f1).kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {}, closed=True)"
+reveal_type(Sneaky(f2, 1).kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int}, closed=True)"
+reveal_type(Sneaky(f3, 1).kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {}, closed=True)"
+reveal_type(Sneaky(f4, x=1).kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x': builtins.int}, closed=True)"
+reveal_type(Sneaky(f5, 1).kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int, 'y'?: builtins.int}, closed=True)"
+reveal_type(Sneaky(f5, 1, 2).kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int, 'y'?: builtins.int}, closed=True)"
 reveal_type(Sneaky(f6, x=1).kwargs)  # N: Revealed type is "builtins.dict[builtins.str, builtins.int]"
 reveal_type(Sneaky(f6, x=1, y=2).kwargs)  # N: Revealed type is "builtins.dict[builtins.str, builtins.int]"
 reveal_type(Sneaky(f7, 1, y='').kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int})"
 reveal_type(Sneaky(f8, 1, y='').kwargs)  # N: Revealed type is "builtins.dict[builtins.str, builtins.str]"
-reveal_type(Sneaky(f9, 1, y=0).kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int, 'y': builtins.int, 'z'?: builtins.str})"
-reveal_type(Sneaky(f9, 1, y=0, z='').kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int, 'y': builtins.int, 'z'?: builtins.str})"
+reveal_type(Sneaky(f9, 1, y=0).kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int, 'y': builtins.int, 'z'?: builtins.str}, closed=True)"
+reveal_type(Sneaky(f9, 1, y=0, z='').kwargs)  # N: Revealed type is "TypedDict('builtins.dict', {'x'?: builtins.int, 'y': builtins.int, 'z'?: builtins.str}, closed=True)"
 [builtins fixtures/paramspec.pyi]