Initial attempt at fixing issue-17706

mypy/dmypy/client.py

@@ -19,6 +19,7 @@
 from mypy.dmypy_os import alive, kill
 from mypy.dmypy_util import DEFAULT_STATUS_FILE, receive, send
 from mypy.ipc import IPCClient, IPCException
+from mypy.main import RECURSION_LIMIT
 from mypy.util import check_python_version, get_terminal_width, should_force_color
 from mypy.version import __version__
 
@@ -267,6 +268,10 @@ class BadStatus(Exception):
 def main(argv: list[str]) -> None:
     """The code is top-down."""
     check_python_version("dmypy")
+
+    # set recursion limit consistent with mypy/main.py
+    sys.setrecursionlimit(RECURSION_LIMIT)
+
     args = parser.parse_args(argv)
     if not args.action:
         parser.print_usage()

mypy/fastparse.py

@@ -240,6 +240,29 @@ def parse(
             strip_function_bodies=strip_function_bodies,
             path=fnam,
         ).visit(ast)
+
+    except RecursionError as e:
+        # For very complex expressions it is possible to hit recursion limit
+        # before reaching a leaf node.
+        # Should reject at top level instead at bottom, since bottom would already
+        # be at the threshold of the recursion limit, and may fail again later.
+        # E.G. x1+x2+x3+...+xn -> BinOp(left=BinOp(left=BinOp(left=...
+        try:
+            # But to prove that is the cause of this particular recursion error,
+            # try to walk the tree using builtin visitor
+            ast3.NodeVisitor().visit(ast)
+        except RecursionError:
+            errors.report(
+                -1, -1, "Source expression too complex to parse", blocker=False, code=codes.MISC
+            )
+
+            tree = MypyFile([], [], False, {})
+
+        else:
+            # re-raise original recursion error if it *can* be unparsed,
+            # maybe this is some other issue that shouldn't be silenced/misdirected
+            raise e
+
     except SyntaxError as e:
         # alias to please mypyc
         is_py38_or_earlier = sys.version_info < (3, 9)
@@ -414,6 +437,7 @@ def visit(self, node: AST | None) -> Any:
             method = "visit_" + node.__class__.__name__
             visitor = getattr(self, method)
             self.visitor_cache[typeobj] = visitor
+
         return visitor(node)
 
     def set_line(self, node: N, n: AstNode) -> N:

mypy/main.py

@@ -29,6 +29,7 @@
 
 orig_stat: Final = os.stat
 MEM_PROFILE: Final = False  # If True, dump memory profile
+RECURSION_LIMIT: Final = 2**14
 
 
 def stat_proxy(path: str) -> os.stat_result:
@@ -63,7 +64,7 @@ def main(
     util.check_python_version("mypy")
     t0 = time.time()
     # To log stat() calls: os.stat = stat_proxy
-    sys.setrecursionlimit(2**14)
+    sys.setrecursionlimit(RECURSION_LIMIT)
     if args is None:
         args = sys.argv[1:]