Make PEP 695 constructs give a reasonable error message

Mypy does not yet support PEP 695

Fixes python#16011, linking python#15238

mypy/fastparse.py

@@ -144,6 +144,11 @@ 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
@@ -936,6 +941,14 @@ def do_func_def(
                 arg_types = [AnyType(TypeOfAny.from_error)] * len(args)
                 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,
+                )
+
             arg_types = [a.type_annotation for a in args]
             return_type = TypeConverter(
                 self.errors, line=n.returns.lineno if n.returns else lineno
@@ -1110,6 +1123,14 @@ 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]
 
+        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,
+            )
+
         cdef = ClassDef(
             n.name,
             self.as_required_block(n.body),
@@ -1717,6 +1738,16 @@ 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)
+
 
 class TypeConverter:
     def __init__(

mypy/test/helpers.py

@@ -241,7 +241,9 @@ def num_skipped_suffix_lines(a1: list[str], a2: list[str]) -> int:
 
 
 def testfile_pyversion(path: str) -> tuple[int, int]:
-    if path.endswith("python311.test"):
+    if path.endswith("python312.test"):
+        return 3, 12
+    elif path.endswith("python311.test"):
         return 3, 11
     elif path.endswith("python310.test"):
         return 3, 10

mypy/test/testcheck.py

@@ -43,6 +43,8 @@
     typecheck_files.remove("check-python310.test")
 if sys.version_info < (3, 11):
     typecheck_files.remove("check-python311.test")
+if sys.version_info < (3, 12):
+    typecheck_files.remove("check-python312.test")
 
 # Special tests for platforms with case-insensitive filesystems.
 if sys.platform not in ("darwin", "win32"):

test-data/unit/check-python312.test

@@ -0,0 +1,35 @@
+[case test695TypeAlias]
+type MyInt = int  # E: PEP 695 type aliases are not yet supported
+
+def f(x: MyInt) -> MyInt:
+    return reveal_type(x)  # N: Revealed type is "builtins.int"
+
+type MyList[T] = list[T]  # E: PEP 695 type aliases are not yet supported \
+                          # E: Name "T" is not defined
+
+def g(x: MyList[int]) -> MyList[int]:  # E: Variable "__main__.MyList" is not valid as a type \
+                                       # N: See https://mypy.readthedocs.io/en/stable/common_issues.html#variables-vs-type-aliases
+    return reveal_type(x)  # N: Revealed type is "MyList?[builtins.int]"
+
+[case test695Class]
+class MyGen[T]:  # E: PEP 695 generics are not yet supported
+    def __init__(self, x: T) -> None:  # E: Name "T" is not defined
+        self.x = x
+
+def f(x: MyGen[int]):  # E: "MyGen" expects no type arguments, but 1 given
+    reveal_type(x.x)  # N: Revealed type is "Any"
+
+[case test695Function]
+def f[T](x: T) -> T:  # E: PEP 695 generics are not yet supported \
+                      # E: Name "T" is not defined
+    return reveal_type(x)  # N: Revealed type is "Any"
+
+reveal_type(f(1))  # N: Revealed type is "Any"
+
+async def g[T](x: T) -> T:  # E: PEP 695 generics are not yet supported \
+                            # E: Name "T" is not defined
+    return reveal_type(x)  # N: Revealed type is "Any"
+
+reveal_type(g(1))  # E: Value of type "Coroutine[Any, Any, Any]" must be used \
+                   # N: Are you missing an await? \
+                   # N: Revealed type is "typing.Coroutine[Any, Any, Any]"