add fuzzer for libstdc++ internal unicode iterator

CMakeLists.txt

@@ -23,6 +23,7 @@ set(fuzzersrcfiles
     format/one-arg.cpp
     format/one-warg.cpp
     format/two-args.cpp
+    libstdcpp/unicode-utf-iterator.cpp
     regex/regex.cpp
     set_like/set_differential.cpp
 )

libstdcpp/unicode-utf-iterator.cpp

@@ -0,0 +1,78 @@
+// Copyright (c) 2024, Paul Dreik
+// Licensed under Boost software license 1.0
+// SPDX-License-Identifier: BSL-1.0
+
+#include <cstddef>
+#include <cstdint>
+#include <span>
+#include <string_view>
+
+#include <bits/unicode.h>
+
+#include "fuzzcombine.h"
+
+namespace {
+template<typename CharType, typename View>
+constexpr inline auto
+make_view(View&& view)
+{
+  return std::__unicode::_Utf_view<CharType, View>{ std::forward<View>(view) };
+}
+
+/*
+ * this iterates over data, interpreting it as utf data.
+ * the point is to exercise std::__unicode::_Utf_view for a variety
+ * of input and output types
+ */
+template<typename CharType>
+[[clang::optnone]] int
+blah(auto data)
+{
+  auto v = make_view<CharType>(data);
+
+  [[maybe_unused]] auto count = 0u;
+  [[maybe_unused]] unsigned int tmp{};
+  for (auto e : v) {
+    tmp += e;
+    ++count;
+  }
+
+  // could anything be asserted by count vs. data.size()?
+  //  assert(count <= data.size());
+
+  // this does not cause a runtime error, the iterator class protects against
+  // misuse
+  [[maybe_unused]] auto illegal = *v.end();
+  return 0;
+}
+}
+
+extern "C" [[clang::optnone]] int
+LLVMFuzzerTestOneInput(const uint8_t* data, size_t size)
+{
+  FuzzCombiner fuzzdata(data, size);
+
+  using OutputTypes = std::tuple<char8_t, char16_t, char32_t>;
+  using InputTypes = std::tuple< // bool,
+    char,
+    wchar_t,
+    char8_t,
+    char16_t,
+    char32_t,
+    signed char,
+    unsigned char,
+    short,
+    unsigned short,
+    int,
+    unsigned int>;
+  fuzzdata.combine_args<OutputTypes, InputTypes>(
+    []<typename Out, typename In>(Out, In, FuzzCombiner* fd) {
+      if constexpr (sizeof(In) == 1) {
+        blah<Out>(fd->get_remainder_as_span<In>());
+      } else {
+        auto d = fd->get_remainder<In>();
+        blah<Out>(std::span{ d.data(), d.data() + d.size() });
+      }
+    });
+  return 0;
+}