fix a fuzzing issue from a string as a bracket

Author: phlptp

Diff for: include/CLI/impl/Option_inl.hpp

@@ -664,6 +664,29 @@ CLI11_INLINE int Option::_add_result(std::string &&result, std::vector<std::stri
     int result_count = 0;
     if((allow_extra_args_ || get_expected_max() > 1) && !result.empty() && result.front() == '[' &&
        result.back() == ']') {  // this is now a vector string likely from the default or user entry
+        if (result.size() >= 4 && result[1] == '[' && (*(result.end() - 2) == ']'))
+        {
+            //this is an escape clause for odd strings
+            std::string nstrs{ '[' };
+            bool duplicated{ true };
+            for (int ii = 2; ii < result.size() - 2; ii+=2)
+            {
+                if (result[ii] == result[ii + 1])
+                {
+                    nstrs.push_back(result[ii]);
+                }
+                else {
+                    duplicated=false;
+                    break;
+                }
+            }
+            if (duplicated)
+            {
+                res.push_back(std::move(nstrs));
+                ++result_count;
+                return result_count;
+            }
+        }
         result.pop_back();
         result.erase(result.begin());
         bool skipSection{false};

Diff for: tests/FuzzFailTest.cpp

@@ -267,10 +267,10 @@ TEST_CASE("app_roundtrip_custom") {
     CLI::FuzzApp fuzzdata2;
     auto app = fuzzdata.generateApp();
     auto app2 = fuzzdata2.generateApp();
-    int index = GENERATE(range(1, 3));
+    int index = GENERATE(range(1, 4));
     std::string optionString, flagString;
     auto parseData = loadFailureFile("round_trip_custom", index);
-    std::size_t pstring_start{0};
+    std::size_t pstring_start{0}; 
     pstring_start = fuzzdata.add_custom_options(app.get(), parseData);
 
     if(pstring_start > 0) {