Add foundation for TypeVar defaults (PEP 696) (#14872)

Start implementing [PEP 696](https://peps.python.org/pep-0696/) TypeVar
defaults. This PR
* Adds a `default` parameter to `TypeVarLikeExpr` and `TypeVarLikeType`.
* Updates most visitors to account for the new `default` parameter.
* Update existing calls to add value for `default` =>
`AnyType(TypeOfAny.from_omitted_generics)`.

A followup PR will update the semantic analyzer and add basic tests for
`TypeVar`, `ParamSpec`, and `TypeVarTuple` calls with a `default`
argument. -> #14873

Ref #14851

mypy/checker.py

@@ -7191,6 +7191,7 @@ def detach_callable(typ: CallableType) -> CallableType:
                 id=var.id,
                 values=var.values,
                 upper_bound=var.upper_bound,
+                default=var.default,
                 variance=var.variance,
             )
         )

mypy/checkexpr.py

@@ -4189,7 +4189,14 @@ def check_lst_expr(self, e: ListExpr | SetExpr | TupleExpr, fullname: str, tag:
         # Used for list and set expressions, as well as for tuples
         # containing star expressions that don't refer to a
         # Tuple. (Note: "lst" stands for list-set-tuple. :-)
-        tv = TypeVarType("T", "T", id=-1, values=[], upper_bound=self.object_type())
+        tv = TypeVarType(
+            "T",
+            "T",
+            id=-1,
+            values=[],
+            upper_bound=self.object_type(),
+            default=AnyType(TypeOfAny.from_omitted_generics),
+        )
         constructor = CallableType(
             [tv],
             [nodes.ARG_STAR],
@@ -4357,8 +4364,22 @@ def visit_dict_expr(self, e: DictExpr) -> Type:
             return dt
 
         # Define type variables (used in constructors below).
-        kt = TypeVarType("KT", "KT", id=-1, values=[], upper_bound=self.object_type())
-        vt = TypeVarType("VT", "VT", id=-2, values=[], upper_bound=self.object_type())
+        kt = TypeVarType(
+            "KT",
+            "KT",
+            id=-1,
+            values=[],
+            upper_bound=self.object_type(),
+            default=AnyType(TypeOfAny.from_omitted_generics),
+        )
+        vt = TypeVarType(
+            "VT",
+            "VT",
+            id=-2,
+            values=[],
+            upper_bound=self.object_type(),
+            default=AnyType(TypeOfAny.from_omitted_generics),
+        )
 
         # Collect function arguments, watching out for **expr.
         args: list[Expression] = []
@@ -4722,7 +4743,14 @@ def check_generator_or_comprehension(
 
             # Infer the type of the list comprehension by using a synthetic generic
             # callable type.
-            tv = TypeVarType("T", "T", id=-1, values=[], upper_bound=self.object_type())
+            tv = TypeVarType(
+                "T",
+                "T",
+                id=-1,
+                values=[],
+                upper_bound=self.object_type(),
+                default=AnyType(TypeOfAny.from_omitted_generics),
+            )
             tv_list: list[Type] = [tv]
             constructor = CallableType(
                 tv_list,
@@ -4742,8 +4770,22 @@ def visit_dictionary_comprehension(self, e: DictionaryComprehension) -> Type:
 
             # Infer the type of the list comprehension by using a synthetic generic
             # callable type.
-            ktdef = TypeVarType("KT", "KT", id=-1, values=[], upper_bound=self.object_type())
-            vtdef = TypeVarType("VT", "VT", id=-2, values=[], upper_bound=self.object_type())
+            ktdef = TypeVarType(
+                "KT",
+                "KT",
+                id=-1,
+                values=[],
+                upper_bound=self.object_type(),
+                default=AnyType(TypeOfAny.from_omitted_generics),
+            )
+            vtdef = TypeVarType(
+                "VT",
+                "VT",
+                id=-2,
+                values=[],
+                upper_bound=self.object_type(),
+                default=AnyType(TypeOfAny.from_omitted_generics),
+            )
             constructor = CallableType(
                 [ktdef, vtdef],
                 [nodes.ARG_POS, nodes.ARG_POS],
@@ -5264,6 +5306,18 @@ def visit_callable_type(self, t: CallableType) -> bool:
             return False
         return super().visit_callable_type(t)
 
+    def visit_type_var(self, t: TypeVarType) -> bool:
+        default = [t.default] if t.has_default() else []
+        return self.query_types([t.upper_bound, *default] + t.values)
+
+    def visit_param_spec(self, t: ParamSpecType) -> bool:
+        default = [t.default] if t.has_default() else []
+        return self.query_types([t.upper_bound, *default])
+
+    def visit_type_var_tuple(self, t: TypeVarTupleType) -> bool:
+        default = [t.default] if t.has_default() else []
+        return self.query_types([t.upper_bound, *default])
+
 
 def has_coroutine_decorator(t: Type) -> bool:
     """Whether t came from a function decorated with `@coroutine`."""

mypy/copytype.py

@@ -72,7 +72,9 @@ def visit_type_var(self, t: TypeVarType) -> ProperType:
         return self.copy_common(t, t.copy_modified())
 
     def visit_param_spec(self, t: ParamSpecType) -> ProperType:
-        dup = ParamSpecType(t.name, t.fullname, t.id, t.flavor, t.upper_bound, prefix=t.prefix)
+        dup = ParamSpecType(
+            t.name, t.fullname, t.id, t.flavor, t.upper_bound, t.default, prefix=t.prefix
+        )
         return self.copy_common(t, dup)
 
     def visit_parameters(self, t: Parameters) -> ProperType:
@@ -86,7 +88,9 @@ def visit_parameters(self, t: Parameters) -> ProperType:
         return self.copy_common(t, dup)
 
     def visit_type_var_tuple(self, t: TypeVarTupleType) -> ProperType:
-        dup = TypeVarTupleType(t.name, t.fullname, t.id, t.upper_bound, t.tuple_fallback)
+        dup = TypeVarTupleType(
+            t.name, t.fullname, t.id, t.upper_bound, t.tuple_fallback, t.default
+        )
         return self.copy_common(t, dup)
 
     def visit_unpack_type(self, t: UnpackType) -> ProperType:

mypy/expandtype.py

@@ -130,14 +130,7 @@ def freshen_function_type_vars(callee: F) -> F:
         tvs = []
         tvmap: dict[TypeVarId, Type] = {}
         for v in callee.variables:
-            if isinstance(v, TypeVarType):
-                tv: TypeVarLikeType = TypeVarType.new_unification_variable(v)
-            elif isinstance(v, TypeVarTupleType):
-                assert isinstance(v, TypeVarTupleType)
-                tv = TypeVarTupleType.new_unification_variable(v)
-            else:
-                assert isinstance(v, ParamSpecType)
-                tv = ParamSpecType.new_unification_variable(v)
+            tv = v.new_unification_variable(v)
             tvs.append(tv)
             tvmap[v.id] = tv
         fresh = expand_type(callee, tvmap).copy_modified(variables=tvs)

mypy/fixup.py

@@ -171,17 +171,21 @@ def visit_class_def(self, c: ClassDef) -> None:
                 for value in v.values:
                     value.accept(self.type_fixer)
             v.upper_bound.accept(self.type_fixer)
+            v.default.accept(self.type_fixer)
 
     def visit_type_var_expr(self, tv: TypeVarExpr) -> None:
         for value in tv.values:
             value.accept(self.type_fixer)
         tv.upper_bound.accept(self.type_fixer)
+        tv.default.accept(self.type_fixer)
 
     def visit_paramspec_expr(self, p: ParamSpecExpr) -> None:
         p.upper_bound.accept(self.type_fixer)
+        p.default.accept(self.type_fixer)
 
     def visit_type_var_tuple_expr(self, tv: TypeVarTupleExpr) -> None:
         tv.upper_bound.accept(self.type_fixer)
+        tv.default.accept(self.type_fixer)
 
     def visit_var(self, v: Var) -> None:
         if self.current_info is not None:
@@ -303,14 +307,16 @@ def visit_type_var(self, tvt: TypeVarType) -> None:
         if tvt.values:
             for vt in tvt.values:
                 vt.accept(self)
-        if tvt.upper_bound is not None:
-            tvt.upper_bound.accept(self)
+        tvt.upper_bound.accept(self)
+        tvt.default.accept(self)
 
     def visit_param_spec(self, p: ParamSpecType) -> None:
         p.upper_bound.accept(self)
+        p.default.accept(self)
 
     def visit_type_var_tuple(self, t: TypeVarTupleType) -> None:
         t.upper_bound.accept(self)
+        t.default.accept(self)
 
     def visit_unpack_type(self, u: UnpackType) -> None:
         u.type.accept(self)

mypy/indirection.py

@@ -64,13 +64,13 @@ def visit_deleted_type(self, t: types.DeletedType) -> set[str]:
         return set()
 
     def visit_type_var(self, t: types.TypeVarType) -> set[str]:
-        return self._visit(t.values) | self._visit(t.upper_bound)
+        return self._visit(t.values) | self._visit(t.upper_bound) | self._visit(t.default)
 
     def visit_param_spec(self, t: types.ParamSpecType) -> set[str]:
-        return set()
+        return self._visit(t.upper_bound) | self._visit(t.default)
 
     def visit_type_var_tuple(self, t: types.TypeVarTupleType) -> set[str]:
-        return self._visit(t.upper_bound)
+        return self._visit(t.upper_bound) | self._visit(t.default)
 
     def visit_unpack_type(self, t: types.UnpackType) -> set[str]:
         return t.type.accept(self)

mypy/nodes.py

@@ -2439,26 +2439,35 @@ class TypeVarLikeExpr(SymbolNode, Expression):
     Note that they are constructed by the semantic analyzer.
     """
 
-    __slots__ = ("_name", "_fullname", "upper_bound", "variance")
+    __slots__ = ("_name", "_fullname", "upper_bound", "default", "variance")
 
     _name: str
     _fullname: str
     # Upper bound: only subtypes of upper_bound are valid as values. By default
     # this is 'object', meaning no restriction.
     upper_bound: mypy.types.Type
+    # Default: used to resolve the TypeVar if the default is not explicitly given.
+    # By default this is 'AnyType(TypeOfAny.from_omitted_generics)'. See PEP 696.
+    default: mypy.types.Type
     # Variance of the type variable. Invariant is the default.
     # TypeVar(..., covariant=True) defines a covariant type variable.
     # TypeVar(..., contravariant=True) defines a contravariant type
     # variable.
     variance: int
 
     def __init__(
-        self, name: str, fullname: str, upper_bound: mypy.types.Type, variance: int = INVARIANT
+        self,
+        name: str,
+        fullname: str,
+        upper_bound: mypy.types.Type,
+        default: mypy.types.Type,
+        variance: int = INVARIANT,
     ) -> None:
         super().__init__()
         self._name = name
         self._fullname = fullname
         self.upper_bound = upper_bound
+        self.default = default
         self.variance = variance
 
     @property
@@ -2496,9 +2505,10 @@ def __init__(
         fullname: str,
         values: list[mypy.types.Type],
         upper_bound: mypy.types.Type,
+        default: mypy.types.Type,
         variance: int = INVARIANT,
     ) -> None:
-        super().__init__(name, fullname, upper_bound, variance)
+        super().__init__(name, fullname, upper_bound, default, variance)
         self.values = values
 
     def accept(self, visitor: ExpressionVisitor[T]) -> T:
@@ -2511,6 +2521,7 @@ def serialize(self) -> JsonDict:
             "fullname": self._fullname,
             "values": [t.serialize() for t in self.values],
             "upper_bound": self.upper_bound.serialize(),
+            "default": self.default.serialize(),
             "variance": self.variance,
         }
 
@@ -2522,6 +2533,7 @@ def deserialize(cls, data: JsonDict) -> TypeVarExpr:
             data["fullname"],
             [mypy.types.deserialize_type(v) for v in data["values"]],
             mypy.types.deserialize_type(data["upper_bound"]),
+            mypy.types.deserialize_type(data["default"]),
             data["variance"],
         )
 
@@ -2540,6 +2552,7 @@ def serialize(self) -> JsonDict:
             "name": self._name,
             "fullname": self._fullname,
             "upper_bound": self.upper_bound.serialize(),
+            "default": self.default.serialize(),
             "variance": self.variance,
         }
 
@@ -2550,6 +2563,7 @@ def deserialize(cls, data: JsonDict) -> ParamSpecExpr:
             data["name"],
             data["fullname"],
             mypy.types.deserialize_type(data["upper_bound"]),
+            mypy.types.deserialize_type(data["default"]),
             data["variance"],
         )
 
@@ -2569,9 +2583,10 @@ def __init__(
         fullname: str,
         upper_bound: mypy.types.Type,
         tuple_fallback: mypy.types.Instance,
+        default: mypy.types.Type,
         variance: int = INVARIANT,
     ) -> None:
-        super().__init__(name, fullname, upper_bound, variance)
+        super().__init__(name, fullname, upper_bound, default, variance)
         self.tuple_fallback = tuple_fallback
 
     def accept(self, visitor: ExpressionVisitor[T]) -> T:
@@ -2584,6 +2599,7 @@ def serialize(self) -> JsonDict:
             "fullname": self._fullname,
             "upper_bound": self.upper_bound.serialize(),
             "tuple_fallback": self.tuple_fallback.serialize(),
+            "default": self.default.serialize(),
             "variance": self.variance,
         }
 
@@ -2595,6 +2611,7 @@ def deserialize(cls, data: JsonDict) -> TypeVarTupleExpr:
             data["fullname"],
             mypy.types.deserialize_type(data["upper_bound"]),
             mypy.types.Instance.deserialize(data["tuple_fallback"]),
+            mypy.types.deserialize_type(data["default"]),
             data["variance"],
         )
 

mypy/plugins/attrs.py

@@ -772,9 +772,14 @@ def _add_order(ctx: mypy.plugin.ClassDefContext, adder: MethodAdder) -> None:
         id=-1,
         values=[],
         upper_bound=object_type,
+        default=AnyType(TypeOfAny.from_omitted_generics),
     )
     self_tvar_expr = TypeVarExpr(
-        SELF_TVAR_NAME, ctx.cls.info.fullname + "." + SELF_TVAR_NAME, [], object_type
+        SELF_TVAR_NAME,
+        ctx.cls.info.fullname + "." + SELF_TVAR_NAME,
+        [],
+        object_type,
+        AnyType(TypeOfAny.from_omitted_generics),
     )
     ctx.cls.info.names[SELF_TVAR_NAME] = SymbolTableNode(MDEF, self_tvar_expr)
 

mypy/plugins/dataclasses.py

@@ -254,7 +254,11 @@ def transform(self) -> bool:
             # Type variable for self types in generated methods.
             obj_type = self._api.named_type("builtins.object")
             self_tvar_expr = TypeVarExpr(
-                SELF_TVAR_NAME, info.fullname + "." + SELF_TVAR_NAME, [], obj_type
+                SELF_TVAR_NAME,
+                info.fullname + "." + SELF_TVAR_NAME,
+                [],
+                obj_type,
+                AnyType(TypeOfAny.from_omitted_generics),
             )
             info.names[SELF_TVAR_NAME] = SymbolTableNode(MDEF, self_tvar_expr)
 
@@ -273,6 +277,7 @@ def transform(self) -> bool:
                     id=-1,
                     values=[],
                     upper_bound=obj_type,
+                    default=AnyType(TypeOfAny.from_omitted_generics),
                 )
                 order_return_type = self._api.named_type("builtins.bool")
                 order_args = [