[PEP 696] Report error if using unsupported type parameter defaults (#17876)

We don't yet support default types for type parameters when using the
new type parameter syntax. Generate an error instead of just ignoring
them.

Also make it possible to write Python 3.13 specific tests.

Author: JukkaL

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