[PEP 696] Report error if using unsupported type parameter defaults

mypy/fastparse.py

@@ -1198,6 +1198,13 @@ def translate_type_params(self, type_params: list[Any]) -> list[TypeParam]:
         for p in type_params:
             bound = None
             values: list[Type] = []
+            if sys.version_info >= (3, 13) and p.default_value is not None:
+                self.fail(
+                    message_registry.TYPE_PARAM_DEFAULT_NOT_SUPPORTED,
+                    p.lineno,
+                    p.col_offset,
+                    blocker=False,
+                )
             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]

mypy/message_registry.py

@@ -362,3 +362,8 @@ def with_additional_msg(self, info: str) -> ErrorMessage:
 TYPE_ALIAS_WITH_AWAIT_EXPRESSION: Final = ErrorMessage(
     "Await expression cannot be used within a type alias", codes.SYNTAX
 )
+
+TYPE_PARAM_DEFAULT_NOT_SUPPORTED: Final = ErrorMessage(
+    "Type parameter default types not supported when using Python 3.12 type parameter syntax",
+    codes.MISC,
+)

mypy/test/helpers.py

@@ -256,16 +256,9 @@ def local_sys_path_set() -> Iterator[None]:
 
 
 def testfile_pyversion(path: str) -> tuple[int, int]:
-    if path.endswith("python312.test"):
-        return 3, 12
-    elif path.endswith("python311.test"):
-        return 3, 11
-    elif path.endswith("python310.test"):
-        return 3, 10
-    elif path.endswith("python39.test"):
-        return 3, 9
-    elif path.endswith("python38.test"):
-        return 3, 8
+    m = re.search(r"python3([0-9]+)\.test$", path)
+    if m:
+        return 3, int(m.group(1))
     else:
         return defaults.PYTHON3_VERSION
 

mypy/test/testcheck.py

@@ -45,6 +45,8 @@
     typecheck_files.remove("check-python311.test")
 if sys.version_info < (3, 12):
     typecheck_files.remove("check-python312.test")
+if sys.version_info < (3, 13):
+    typecheck_files.remove("check-python313.test")
 
 # Special tests for platforms with case-insensitive filesystems.
 if sys.platform not in ("darwin", "win32"):

test-data/unit/check-python313.test

@@ -0,0 +1,11 @@
+[case testPEP695TypeParameterDefaultNotSupported]
+class C[T = None]:  # E: Type parameter default types not supported when using Python 3.12 type parameter syntax
+    pass
+
+def f[T = list[int]]() -> None:  # E: Type parameter default types not supported when using Python 3.12 type parameter syntax
+    pass
+
+def g[**P = [int, str]]() -> None:  # E: Type parameter default types not supported when using Python 3.12 type parameter syntax
+    pass
+
+type A[T, S = int, U = str] = list[T]  # E: Type parameter default types not supported when using Python 3.12 type parameter syntax