testsuite: Add testcase for dangling pointer equality bogus warning [PR104492]

On Wed, Apr 27, 2022 at 12:02:33PM +0200, Richard Biener wrote:
> I did that but the reduction result did not resemble the same failure
> mode.  I've failed to manually construct a testcase as well.  Possibly
> a testcase using libstdc++ but less Qt internals might be possible.

Here is a testcase that I've managed to reduce, FAILs with:
FAIL: g++.dg/warn/pr104492.C  -std=gnu++14  (test for bogus messages, line 111)
FAIL: g++.dg/warn/pr104492.C  -std=gnu++17  (test for bogus messages, line 111)
FAIL: g++.dg/warn/pr104492.C  -std=gnu++20  (test for bogus messages, line 111)
on both x86_64-linux and i686-linux without your commit and passes with it.

2022-04-27  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/104492
	* g++.dg/warn/pr104492.C: New test.

gcc/testsuite/g++.dg/warn/pr104492.C

@@ -0,0 +1,115 @@
+// PR middle-end/104492
+// { dg-do compile { target c++14 } }
+// { dg-options "-O3 -Wall" }
+
+namespace std {
+typedef decltype (sizeof 0) size_t;
+template <typename _Tp, _Tp __v> struct integral_constant {
+  static constexpr _Tp value = __v;
+};
+template <typename _Tp, typename _Up>
+struct is_same : integral_constant<bool, __is_same(_Tp, _Up)> {};
+template <bool, typename> struct enable_if;
+template <typename _Tp> _Tp forward;
+}
+class QString;
+struct random_access_iterator_tag {};
+template <typename> struct iterator_traits {
+  typedef random_access_iterator_tag iterator_category;
+};
+template <typename _Iter>
+typename iterator_traits<_Iter>::iterator_category __iterator_category(_Iter);
+namespace __gnu_cxx {
+namespace __ops {
+template <typename> struct _Iter_equals_val {
+  template <typename _Iterator> bool operator()(_Iterator);
+};
+template <typename _Value> _Iter_equals_val<_Value> __iter_equals_val(_Value);
+}
+}
+namespace std {
+template <typename _RandomAccessIterator, typename _Predicate>
+_RandomAccessIterator __find_if(_RandomAccessIterator __first,
+				_RandomAccessIterator __last, _Predicate __pred,
+				random_access_iterator_tag) {
+  if (__pred(__first))
+    return __first;
+  return __last;
+}
+template <typename _Iterator, typename _Predicate>
+_Iterator __find_if(_Iterator __first, _Iterator __last, _Predicate __pred) {
+  return __find_if(__first, __last, __pred, __iterator_category(__first));
+}
+template <typename _Tp, size_t _Nm> _Tp *begin(_Tp (&__arr)[_Nm]) {
+  return __arr;
+}
+template <typename _Tp, size_t _Nm> _Tp *end(_Tp (&__arr)[_Nm]) {
+  return __arr + _Nm;
+}
+template <typename _InputIterator, typename _Tp>
+_InputIterator find(_InputIterator __first, _InputIterator __last, _Tp __val) {
+  return __find_if(__first, __last, __gnu_cxx::__ops::__iter_equals_val(__val));
+}
+}
+struct QStringView {
+  template <typename T>
+  using if_compatible_qstring_like = std::enable_if<std::is_same<T, QString>::value, bool>;
+  template <typename String> QStringView(String);
+};
+template <typename Haystack, typename> struct QStringTokenizerBase {
+  class sentinel {};
+  struct iterator {
+    Haystack operator*();
+    iterator operator++(int);
+    bool operator!=(sentinel);
+  };
+  iterator begin();
+  template <bool = std::is_same<iterator, sentinel>::value> sentinel end();
+};
+namespace QtPrivate {
+namespace Tok {
+template <typename Haystack, typename Needle>
+using TokenizerBase = QStringTokenizerBase<Haystack, Needle>;
+}
+}
+template <typename Haystack, typename Needle>
+struct QStringTokenizer
+    : public QtPrivate::Tok::TokenizerBase<Haystack, Needle> {
+  QStringTokenizer(Haystack, Needle);
+};
+namespace QtPrivate {
+namespace Tok {
+template <typename Haystack, typename Needle>
+using TokenizerResult = QStringTokenizer<Haystack, Needle>;
+}
+}
+template <typename Haystack, typename Needle>
+auto qTokenize(Haystack, Needle)
+    -> decltype(QtPrivate::Tok::TokenizerResult<Haystack, Needle>{
+	std::forward<Haystack>, std::forward<Needle>});
+struct QLatin1String {
+  QLatin1String(const char *) {}
+};
+class QString {};
+class QLibrary {
+  bool isLibrary(const QString &);
+};
+
+bool QLibrary::isLibrary(const QString &fileName)
+{
+    QString completeSuffix = fileName;
+    auto isValidSuffix = [](QStringView s) {
+	const QLatin1String candidates[] = {
+	    QLatin1String("so"),
+	};
+	return std::find(std::begin(candidates), std::end(candidates), s) != std::end(candidates);
+    };
+    auto suffixes = qTokenize(completeSuffix, u'.');
+    auto it = suffixes.begin();
+    const auto end = suffixes.end();
+    while (it != end) {
+	if (isValidSuffix(*it++))	// { dg-bogus "dangling pointer to .candidates. may be used" }
+	  return true;
+    }
+    return false;
+}