Add warning if --force-union-syntax is used

Author: cdce8p

mypy/main.py

@@ -819,6 +819,7 @@ def add_invertible_flag(
     add_invertible_flag(
         "--force-uppercase-builtins", default=False, help=argparse.SUPPRESS, group=none_group
     )
+
     # This flag is deprecated, Mypy only supports Python 3.10+
     add_invertible_flag(
         "--force-union-syntax", default=False, help=argparse.SUPPRESS, group=none_group
@@ -1544,6 +1545,9 @@ def set_strict_flags() -> None:
     if options.force_uppercase_builtins:
         print("Warning: --force-uppercase-builtins is deprecated; mypy only supports Python 3.9+")
 
+    if options.force_union_syntax:
+        print("Warning: --force-union-syntax is deprecated; mypy only supports Python 3.10+")
+
     # Set target.
     if special_opts.modules + special_opts.packages:
         options.build_type = BuildType.MODULE