[libc++][vector<bool>] Tests shrink_to_fit requirement.

vector<bool>'s shrink_to_fit implementation is using the
"swap-to-free-container-resources-trick" which only shrinks when the input
vector is empty. Since the request to shrink_to_fit is non-binding, this
is a valid implementation. It is not a high-quality implementation. Since
vector<bool> is not a very popular container the implementation has not
been changed and only a test to validate the non-growing property has been
added.

This was discovered while investigating #95161

Author: mordante

libcxx/test/std/containers/sequences/vector.bool/shrink_to_fit.pass.cpp

@@ -39,9 +39,46 @@ TEST_CONSTEXPR_CXX20 bool tests()
     return true;
 }
 
+#if TEST_STD_VER >= 23
+std::size_t min_bytes = 1000;
+
+template <typename T>
+struct increasing_allocator {
+  using value_type       = T;
+  increasing_allocator() = default;
+  template <typename U>
+  increasing_allocator(const increasing_allocator<U>&) noexcept {}
+  std::allocation_result<T*> allocate_at_least(std::size_t n) {
+    std::size_t allocation_amount = n * sizeof(T);
+    if (allocation_amount < min_bytes)
+      allocation_amount = min_bytes;
+    min_bytes += 1000;
+    return {static_cast<T*>(::operator new(allocation_amount)), allocation_amount / sizeof(T)};
+  }
+  T* allocate(std::size_t n) { return allocate_at_least(n).ptr; }
+  void deallocate(T* p, std::size_t) noexcept { ::operator delete(static_cast<void*>(p)); }
+};
+
+template <typename T, typename U>
+bool operator==(increasing_allocator<T>, increasing_allocator<U>) {
+  return true;
+}
+
+// https://github.com/llvm/llvm-project/issues/95161
+void test_increasing_allocator() {
+  std::vector<bool, increasing_allocator<bool>> v;
+  v.push_back(1);
+  std::size_t capacity = v.capacity();
+  v.shrink_to_fit();
+  assert(v.capacity() <= capacity);
+  assert(v.size() == 1);
+}
+#endif // TEST_STD_VER >= 23
+
 int main(int, char**)
 {
     tests();
+    test_increasing_allocator();
 #if TEST_STD_VER > 17
     static_assert(tests());
 #endif