[PEP 695] Partial support for new type parameter syntax in Python 3.12

mypy/checker.py

@@ -146,6 +146,7 @@
 from mypy.state import state
 from mypy.subtypes import (
     find_member,
+    infer_class_variances,
     is_callable_compatible,
     is_equivalent,
     is_more_precise,
@@ -2374,7 +2375,7 @@ def visit_class_def(self, defn: ClassDef) -> None:
                     self.allow_abstract_call = old_allow_abstract_call
                 # TODO: Apply the sig to the actual TypeInfo so we can handle decorators
                 # that completely swap out the type.  (e.g. Callable[[Type[A]], Type[B]])
-        if typ.defn.type_vars:
+        if typ.defn.type_vars and typ.defn.type_args is None:
             for base_inst in typ.bases:
                 for base_tvar, base_decl_tvar in zip(
                     base_inst.args, base_inst.type.defn.type_vars
@@ -2396,6 +2397,7 @@ def visit_class_def(self, defn: ClassDef) -> None:
             self.check_protocol_variance(defn)
         if not defn.has_incompatible_baseclass and defn.info.is_enum:
             self.check_enum(defn)
+        infer_class_variances(defn.info)
 
     def check_final_deletable(self, typ: TypeInfo) -> None:
         # These checks are only for mypyc. Only perform some checks that are easier
@@ -2566,6 +2568,9 @@ def check_protocol_variance(self, defn: ClassDef) -> None:
         if they are actually covariant/contravariant, since this may break
         transitivity of subtyping, see PEP 544.
         """
+        if defn.type_args is not None:
+            # Using new-style syntax (PEP 695), so variance will be inferred
+            return
         info = defn.info
         object_type = Instance(info.mro[-1], [])
         tvars = info.defn.type_vars
@@ -3412,8 +3417,8 @@ def check_final(self, s: AssignmentStmt | OperatorAssignmentStmt | AssignmentExp
                 if (
                     lv.node.final_unset_in_class
                     and not lv.node.final_set_in_init
-                    and not self.is_stub
-                    and  # It is OK to skip initializer in stub files.
+                    and not self.is_stub  # It is OK to skip initializer in stub files.
+                    and
                     # Avoid extra error messages, if there is no type in Final[...],
                     # then we already reported the error about missing r.h.s.
                     isinstance(s, AssignmentStmt)

mypy/fastparse.py

@@ -17,6 +17,9 @@
     ARG_POS,
     ARG_STAR,
     ARG_STAR2,
+    PARAM_SPEC_KIND,
+    TYPE_VAR_KIND,
+    TYPE_VAR_TUPLE_KIND,
     ArgKind,
     Argument,
     AssertStmt,
@@ -79,6 +82,8 @@
     TempNode,
     TryStmt,
     TupleExpr,
+    TypeAliasStmt,
+    TypeParam,
     UnaryExpr,
     Var,
     WhileStmt,
@@ -87,7 +92,7 @@
     YieldFromExpr,
     check_arg_names,
 )
-from mypy.options import Options
+from mypy.options import NEW_GENERIC_SYNTAX, Options
 from mypy.patterns import (
     AsPattern,
     ClassPattern,
@@ -144,11 +149,6 @@ def ast3_parse(
 NamedExpr = ast3.NamedExpr
 Constant = ast3.Constant
 
-if sys.version_info >= (3, 12):
-    ast_TypeAlias = ast3.TypeAlias
-else:
-    ast_TypeAlias = Any
-
 if sys.version_info >= (3, 10):
     Match = ast3.Match
     MatchValue = ast3.MatchValue
@@ -171,11 +171,21 @@ def ast3_parse(
     MatchAs = Any
     MatchOr = Any
     AstNode = Union[ast3.expr, ast3.stmt, ast3.ExceptHandler]
+
 if sys.version_info >= (3, 11):
     TryStar = ast3.TryStar
 else:
     TryStar = Any
 
+if sys.version_info >= (3, 12):
+    ast_TypeAlias = ast3.TypeAlias
+    ast_ParamSpec = ast3.ParamSpec
+    ast_TypeVarTuple = ast3.TypeVarTuple
+else:
+    ast_TypeAlias = Any
+    ast_ParamSpec = Any
+    ast_TypeVarTuple = Any
+
 N = TypeVar("N", bound=Node)
 
 # There is no way to create reasonable fallbacks at this stage,
@@ -884,6 +894,8 @@ def do_func_def(
 
         arg_kinds = [arg.kind for arg in args]
         arg_names = [None if arg.pos_only else arg.variable.name for arg in args]
+        # Type parameters, if using new syntax for generics (PEP 695)
+        explicit_type_params: list[TypeParam] | None = None
 
         arg_types: list[Type | None] = []
         if no_type_check:
@@ -937,12 +949,17 @@ def do_func_def(
                 return_type = AnyType(TypeOfAny.from_error)
         else:
             if sys.version_info >= (3, 12) and n.type_params:
-                self.fail(
-                    ErrorMessage("PEP 695 generics are not yet supported", code=codes.VALID_TYPE),
-                    n.type_params[0].lineno,
-                    n.type_params[0].col_offset,
-                    blocker=False,
-                )
+                if NEW_GENERIC_SYNTAX in self.options.enable_incomplete_feature:
+                    explicit_type_params = self.translate_type_params(n.type_params)
+                else:
+                    self.fail(
+                        ErrorMessage(
+                            "PEP 695 generics are not yet supported", code=codes.VALID_TYPE
+                        ),
+                        n.type_params[0].lineno,
+                        n.type_params[0].col_offset,
+                        blocker=False,
+                    )
 
             arg_types = [a.type_annotation for a in args]
             return_type = TypeConverter(
@@ -986,7 +1003,7 @@ def do_func_def(
         self.class_and_function_stack.pop()
         self.class_and_function_stack.append("F")
         body = self.as_required_block(n.body, can_strip=True, is_coroutine=is_coroutine)
-        func_def = FuncDef(n.name, args, body, func_type)
+        func_def = FuncDef(n.name, args, body, func_type, explicit_type_params)
         if isinstance(func_def.type, CallableType):
             # semanal.py does some in-place modifications we want to avoid
             func_def.unanalyzed_type = func_def.type.copy_modified()
@@ -1120,13 +1137,19 @@ def visit_ClassDef(self, n: ast3.ClassDef) -> ClassDef:
         self.class_and_function_stack.append("C")
         keywords = [(kw.arg, self.visit(kw.value)) for kw in n.keywords if kw.arg]
 
+        # Type parameters, if using new syntax for generics (PEP 695)
+        explicit_type_params: list[TypeParam] | None = None
+
         if sys.version_info >= (3, 12) and n.type_params:
-            self.fail(
-                ErrorMessage("PEP 695 generics are not yet supported", code=codes.VALID_TYPE),
-                n.type_params[0].lineno,
-                n.type_params[0].col_offset,
-                blocker=False,
-            )
+            if NEW_GENERIC_SYNTAX in self.options.enable_incomplete_feature:
+                explicit_type_params = self.translate_type_params(n.type_params)
+            else:
+                self.fail(
+                    ErrorMessage("PEP 695 generics are not yet supported", code=codes.VALID_TYPE),
+                    n.type_params[0].lineno,
+                    n.type_params[0].col_offset,
+                    blocker=False,
+                )
 
         cdef = ClassDef(
             n.name,
@@ -1135,6 +1158,7 @@ def visit_ClassDef(self, n: ast3.ClassDef) -> ClassDef:
             self.translate_expr_list(n.bases),
             metaclass=dict(keywords).get("metaclass"),
             keywords=keywords,
+            type_args=explicit_type_params,
         )
         cdef.decorators = self.translate_expr_list(n.decorator_list)
         # Set lines to match the old mypy 0.700 lines, in order to keep
@@ -1150,6 +1174,24 @@ def visit_ClassDef(self, n: ast3.ClassDef) -> ClassDef:
         self.class_and_function_stack.pop()
         return cdef
 
+    def translate_type_params(self, type_params: list[Any]) -> list[TypeParam]:
+        explicit_type_params = []
+        for p in type_params:
+            bound = None
+            values: list[Type] = []
+            if isinstance(p, ast_ParamSpec):  # type: ignore[misc]
+                explicit_type_params.append(TypeParam(p.name, PARAM_SPEC_KIND, None, []))
+            elif isinstance(p, ast_TypeVarTuple):  # type: ignore[misc]
+                explicit_type_params.append(TypeParam(p.name, TYPE_VAR_TUPLE_KIND, None, []))
+            else:
+                if isinstance(p.bound, ast3.Tuple):
+                    conv = TypeConverter(self.errors, line=p.lineno)
+                    values = [conv.visit(t) for t in p.bound.elts]
+                elif p.bound is not None:
+                    bound = TypeConverter(self.errors, line=p.lineno).visit(p.bound)
+                explicit_type_params.append(TypeParam(p.name, TYPE_VAR_KIND, bound, values))
+        return explicit_type_params
+
     # Return(expr? value)
     def visit_Return(self, n: ast3.Return) -> ReturnStmt:
         node = ReturnStmt(self.visit(n.value))
@@ -1735,15 +1777,23 @@ def visit_MatchOr(self, n: MatchOr) -> OrPattern:
         node = OrPattern([self.visit(pattern) for pattern in n.patterns])
         return self.set_line(node, n)
 
-    def visit_TypeAlias(self, n: ast_TypeAlias) -> AssignmentStmt:
-        self.fail(
-            ErrorMessage("PEP 695 type aliases are not yet supported", code=codes.VALID_TYPE),
-            n.lineno,
-            n.col_offset,
-            blocker=False,
-        )
-        node = AssignmentStmt([NameExpr(n.name.id)], self.visit(n.value))
-        return self.set_line(node, n)
+    # TypeAlias(identifier name, type_param* type_params, expr value)
+    def visit_TypeAlias(self, n: ast_TypeAlias) -> TypeAliasStmt | AssignmentStmt:
+        node: TypeAliasStmt | AssignmentStmt
+        if NEW_GENERIC_SYNTAX in self.options.enable_incomplete_feature:
+            type_params = self.translate_type_params(n.type_params)
+            value = self.visit(n.value)
+            node = TypeAliasStmt(self.visit_Name(n.name), type_params, value)
+            return self.set_line(node, n)
+        else:
+            self.fail(
+                ErrorMessage("PEP 695 type aliases are not yet supported", code=codes.VALID_TYPE),
+                n.lineno,
+                n.col_offset,
+                blocker=False,
+            )
+            node = AssignmentStmt([NameExpr(n.name.id)], self.visit(n.value))
+            return self.set_line(node, n)
 
 
 class TypeConverter:

mypy/join.py

@@ -6,7 +6,7 @@
 
 import mypy.typeops
 from mypy.maptype import map_instance_to_supertype
-from mypy.nodes import CONTRAVARIANT, COVARIANT, INVARIANT
+from mypy.nodes import CONTRAVARIANT, COVARIANT, INVARIANT, VARIANCE_NOT_READY
 from mypy.state import state
 from mypy.subtypes import (
     SubtypeContext,
@@ -97,7 +97,7 @@ def join_instances(self, t: Instance, s: Instance) -> ProperType:
                 elif isinstance(sa_proper, AnyType):
                     new_type = AnyType(TypeOfAny.from_another_any, sa_proper)
                 elif isinstance(type_var, TypeVarType):
-                    if type_var.variance == COVARIANT:
+                    if type_var.variance in (COVARIANT, VARIANCE_NOT_READY):
                         new_type = join_types(ta, sa, self)
                         if len(type_var.values) != 0 and new_type not in type_var.values:
                             self.seen_instances.pop()

mypy/nodes.py

@@ -653,6 +653,28 @@ def set_line(
         self.variable.set_line(self.line, self.column, self.end_line, self.end_column)
 
 
+# These specify the kind of a TypeParam
+TYPE_VAR_KIND: Final = 0
+PARAM_SPEC_KIND: Final = 1
+TYPE_VAR_TUPLE_KIND: Final = 2
+
+
+class TypeParam:
+    __slots__ = ("name", "kind", "upper_bound", "values")
+
+    def __init__(
+        self,
+        name: str,
+        kind: int,
+        upper_bound: mypy.types.Type | None,
+        values: list[mypy.types.Type],
+    ) -> None:
+        self.name = name
+        self.kind = kind
+        self.upper_bound = upper_bound
+        self.values = values
+
+
 FUNCITEM_FLAGS: Final = FUNCBASE_FLAGS + [
     "is_overload",
     "is_generator",
@@ -672,6 +694,7 @@ class FuncItem(FuncBase):
         "min_args",  # Minimum number of arguments
         "max_pos",  # Maximum number of positional arguments, -1 if no explicit
         # limit (*args not included)
+        "type_args",  # New-style type parameters (PEP 695)
         "body",  # Body of the function
         "is_overload",  # Is this an overload variant of function with more than
         # one overload variant?
@@ -689,12 +712,14 @@ def __init__(
         arguments: list[Argument] | None = None,
         body: Block | None = None,
         typ: mypy.types.FunctionLike | None = None,
+        type_args: list[TypeParam] | None = None,
     ) -> None:
         super().__init__()
         self.arguments = arguments or []
         self.arg_names = [None if arg.pos_only else arg.variable.name for arg in self.arguments]
         self.arg_kinds: list[ArgKind] = [arg.kind for arg in self.arguments]
         self.max_pos: int = self.arg_kinds.count(ARG_POS) + self.arg_kinds.count(ARG_OPT)
+        self.type_args: list[TypeParam] | None = type_args
         self.body: Block = body or Block([])
         self.type = typ
         self.unanalyzed_type = typ
@@ -761,8 +786,9 @@ def __init__(
         arguments: list[Argument] | None = None,
         body: Block | None = None,
         typ: mypy.types.FunctionLike | None = None,
+        type_args: list[TypeParam] | None = None,
     ) -> None:
-        super().__init__(arguments, body, typ)
+        super().__init__(arguments, body, typ, type_args)
         self._name = name
         self.is_decorated = False
         self.is_conditional = False  # Defined conditionally (within block)?
@@ -1070,6 +1096,7 @@ class ClassDef(Statement):
         "name",
         "_fullname",
         "defs",
+        "type_args",
         "type_vars",
         "base_type_exprs",
         "removed_base_type_exprs",
@@ -1089,6 +1116,9 @@ class ClassDef(Statement):
     name: str  # Name of the class without module prefix
     _fullname: str  # Fully qualified name of the class
     defs: Block
+    # New-style type parameters (PEP 695), unanalyzed
+    type_args: list[TypeParam] | None
+    # Semantically analyzed type parameters (all syntax variants)
     type_vars: list[mypy.types.TypeVarLikeType]
     # Base class expressions (not semantically analyzed -- can be arbitrary expressions)
     base_type_exprs: list[Expression]
@@ -1111,12 +1141,14 @@ def __init__(
         base_type_exprs: list[Expression] | None = None,
         metaclass: Expression | None = None,
         keywords: list[tuple[str, Expression]] | None = None,
+        type_args: list[TypeParam] | None = None,
     ) -> None:
         super().__init__()
         self.name = name
         self._fullname = ""
         self.defs = defs
         self.type_vars = type_vars or []
+        self.type_args = type_args
         self.base_type_exprs = base_type_exprs or []
         self.removed_base_type_exprs = []
         self.info = CLASSDEF_NO_INFO
@@ -1607,6 +1639,25 @@ def accept(self, visitor: StatementVisitor[T]) -> T:
         return visitor.visit_match_stmt(self)
 
 
+class TypeAliasStmt(Statement):
+    __slots__ = ("name", "type_args", "value")
+
+    __match_args__ = ("name", "type_args", "value")
+
+    name: NameExpr
+    type_args: list[TypeParam]
+    value: Expression  # Will get translated into a type
+
+    def __init__(self, name: NameExpr, type_args: list[TypeParam], value: Expression) -> None:
+        super().__init__()
+        self.name = name
+        self.type_args = type_args
+        self.value = value
+
+    def accept(self, visitor: StatementVisitor[T]) -> T:
+        return visitor.visit_type_alias_stmt(self)
+
+
 # Expressions
 
 
@@ -2442,6 +2493,7 @@ def accept(self, visitor: ExpressionVisitor[T]) -> T:
 INVARIANT: Final = 0
 COVARIANT: Final = 1
 CONTRAVARIANT: Final = 2
+VARIANCE_NOT_READY: Final = 3  # Variance hasn't been inferred (using Python 3.12 syntax)
 
 
 class TypeVarLikeExpr(SymbolNode, Expression):

mypy/options.py

@@ -73,7 +73,8 @@ class BuildType:
 TYPE_VAR_TUPLE: Final = "TypeVarTuple"
 UNPACK: Final = "Unpack"
 PRECISE_TUPLE_TYPES: Final = "PreciseTupleTypes"
-INCOMPLETE_FEATURES: Final = frozenset((PRECISE_TUPLE_TYPES,))
+NEW_GENERIC_SYNTAX: Final = "NewGenericSyntax"
+INCOMPLETE_FEATURES: Final = frozenset((PRECISE_TUPLE_TYPES, NEW_GENERIC_SYNTAX))
 COMPLETE_FEATURES: Final = frozenset((TYPE_VAR_TUPLE, UNPACK))