fix: a potential stack overflow error (#665)

* Fix a potential stack overflow error if nameless subcommands have fallthough

* style: pre-commit.ci fixes

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>

examples/CMakeLists.txt

@@ -247,3 +247,8 @@ set_property(TEST retired_deprecated PROPERTY PASS_REGULAR_EXPRESSION "deprecate
 add_cli_exe(custom_parse custom_parse.cpp)
 add_test(NAME cp_test COMMAND custom_parse --dv 1.7)
 set_property(TEST cp_test PROPERTY PASS_REGULAR_EXPRESSION "called correct")
+
+#------------------------------------------------
+# This executable is for manual testing and is expected to change regularly
+
+add_cli_exe(tester testEXE.cpp)

examples/testEXE.cpp

@@ -0,0 +1,26 @@
+// Copyright (c) 2017-2021, University of Cincinnati, developed by Henry Schreiner
+// under NSF AWARD 1414736 and by the respective contributors.
+// All rights reserved.
+//
+// SPDX-License-Identifier: BSD-3-Clause
+
+// Code modified from https://github.com/CLIUtils/CLI11/issues/559
+
+#include <CLI/CLI.hpp>
+#include <iostream>
+#include <string>
+
+int main(int argc, const char *argv[]) {
+
+    int logLevel{0};
+    CLI::App app{"Test App"};
+
+    app.add_option("-v", logLevel, "level");
+
+    auto subcom = app.add_subcommand("sub", "")->fallthrough();
+    subcom->preparse_callback([&app](size_t) { app.get_subcommand("sub")->add_option_group("group"); });
+
+    CLI11_PARSE(app, argc, argv);
+
+    std::cout << "level: " << logLevel << std::endl;
+}

include/CLI/App.hpp

@@ -2732,13 +2732,16 @@ class App {
                     }
                 }
             }
-            // If a subcommand, try the main command
+
-            if(parent_ != nullptr && fallthrough_)
+            // don't capture missing if this is a nameless subcommand and nameless subcommands can't fallthrough
-                return _get_fallthrough_parent()->_parse_arg(args, current_type);
-            // don't capture missing if this is a nameless subcommand
             if(parent_ != nullptr && name_.empty()) {
                 return false;
             }
+
+            // If a subcommand, try the main command
+            if(parent_ != nullptr && fallthrough_)
+                return _get_fallthrough_parent()->_parse_arg(args, current_type);
+
             // Otherwise, add to missing
             args.pop_back();
             _move_to_missing(current_type, current);