Fix iterator UB in ARGS_NOEXCEPT completion parsing

poc/build_and_run.sh

@@ -0,0 +1,35 @@
+#!/usr/bin/env bash
+# Build PoC against buggy and fixed args.hxx, run both under ASan+UBSan.
+set -u
+HERE="$(cd "$(dirname "$0")" && pwd)"
+cd "$HERE"
+
+# Extract the two args.hxx versions from git history into this directory.
+( cd /mnt/d/args && git show a1407f8^:args.hxx > "$HERE/args_buggy.hxx" )
+( cd /mnt/d/args && git show a1407f8:args.hxx  > "$HERE/args_fixed.hxx" )
+ls -la args_*.hxx
+
+SAN='-fsanitize=address,undefined -fno-sanitize-recover=all -fno-omit-frame-pointer'
+COMMON="-std=c++11 -O0 -g -I. $SAN -Wall"
+
+echo "===== BUILD: buggy ====="
+clang++ $COMMON -DARGS_HXX_PATH='"args_buggy.hxx"' poc_noexcept_iter_oob.cxx -o poc_buggy
+echo "build exit: $?"
+
+echo "===== BUILD: fixed ====="
+clang++ $COMMON -DARGS_HXX_PATH='"args_fixed.hxx"' poc_noexcept_iter_oob.cxx -o poc_fixed
+echo "build exit: $?"
+
+echo
+echo "===== RUN: buggy ====="
+ASAN_OPTIONS=abort_on_error=0:halt_on_error=0:print_stacktrace=1 \
+UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=0 \
+./poc_buggy
+echo "buggy exit: $?"
+
+echo
+echo "===== RUN: fixed ====="
+ASAN_OPTIONS=abort_on_error=0:halt_on_error=0:print_stacktrace=1 \
+UBSAN_OPTIONS=print_stacktrace=1:halt_on_error=0 \
+./poc_fixed
+echo "fixed exit: $?"

poc/poc_noexcept_iter_oob.cxx

@@ -0,0 +1,76 @@
+// PoC for ARGS_NOEXCEPT iterator OOB and dangling-iterator bugs fixed
+// in commit a1407f8. Exercises both sites:
+//   Site 1: ParseArgsValues completion path sets `it = end`, caller does ++it
+//           on an already-end iterator -> OOB increment + OOB read.
+//   Site 2: ParseLong returns an iterator into a local `curArgs` vector that
+//           is destroyed when the function returns -> dangling iterator
+//           returned to ParseCLI and compared with == against outer end.
+//
+// Build:
+//   clang++ -std=c++11 -O0 -g -fsanitize=address,undefined \
+//       -DARGS_HXX_PATH='"args_buggy.hxx"'  poc_noexcept_iter_oob.cxx -o poc_buggy
+//   clang++ -std=c++11 -O0 -g -fsanitize=address,undefined \
+//       -DARGS_HXX_PATH='"args_fixed.hxx"'  poc_noexcept_iter_oob.cxx -o poc_fixed
+//
+// Run:
+//   ./poc_buggy   # expect ASan heap-buffer-overflow + UBSan dangling compare
+//   ./poc_fixed   # expect clean exit
+
+#define ARGS_NOEXCEPT
+
+#ifndef ARGS_HXX_PATH
+#define ARGS_HXX_PATH "args.hxx"
+#endif
+
+#include ARGS_HXX_PATH
+
+#include <cstdio>
+#include <vector>
+#include <string>
+
+int main()
+{
+    // -------- Site 1: ARGS_NOEXCEPT completion on a flag value position --------
+    // cword=2 lands on the empty value position of "-m". On the buggy build
+    // ParseArgsValues sets `it = end` then the caller does `++it` on an
+    // already-end iterator, after which the loop dereferences it.
+    {
+        args::ArgumentParser p("parser");
+        args::CompletionFlag c(p, {"completion"});
+        args::MapFlag<std::string, int> m(
+            p, "mappos", "mappos", {'m', "map"},
+            {{"alpha", 1}, {"beta", 2}});
+
+        p.ParseArgs(std::vector<std::string>{
+            "--completion", "bash", "2", "test", "-m", ""});
+
+        std::fprintf(stderr, "site1 ok: err=%d reply=[%s]\n",
+                     static_cast<int>(p.GetError()),
+                     args::get(c).c_str());
+    }
+
+    // -------- Site 2: Dangling iterator returned across stack frames --------
+    // ParseCLI compares the return value of ParseArgs against std::end(args).
+    // On the buggy build the returned iterator points into a destroyed local
+    // `curArgs` vector inside the completion handler; the == compare is then
+    // a dangling-iterator compare (UB).
+    {
+        args::ArgumentParser p("parser");
+        args::CompletionFlag c(p, {"completion"});
+        args::MapFlag<std::string, int> m(
+            p, "mappos", "mappos", {'m', "map"},
+            {{"alpha", 1}, {"beta", 2}});
+
+        const char *argv[] = {
+            "prog", "--completion", "bash", "2", "test", "-m", ""};
+        const int argc = static_cast<int>(sizeof(argv) / sizeof(argv[0]));
+        p.ParseCLI(argc, argv);
+
+        std::fprintf(stderr, "site2 ok: err=%d reply=[%s]\n",
+                     static_cast<int>(p.GetError()),
+                     args::get(c).c_str());
+    }
+
+    std::fprintf(stderr, "done\n");
+    return 0;
+}