diff --git a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx/string/simulator/Makefile b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx/string/simulator/Makefile
new file mode 100644
index 0000000000000..38cfa81053488
--- /dev/null
+++ b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx/string/simulator/Makefile
@@ -0,0 +1,3 @@
+CXX_SOURCES := main.cpp
+override CXXFLAGS_EXTRAS += -std=c++14
+include Makefile.rules
diff --git a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx/string/simulator/TestDataFormatterLibcxxStringSimulator.py b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx/string/simulator/TestDataFormatterLibcxxStringSimulator.py
new file mode 100644
index 0000000000000..76cd64660aab0
--- /dev/null
+++ b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx/string/simulator/TestDataFormatterLibcxxStringSimulator.py
@@ -0,0 +1,45 @@
+"""
+Test we can understand various layouts of the libc++'s std::string
+"""
+
+
+
+import lldb
+from lldbsuite.test.decorators import *
+from lldbsuite.test.lldbtest import *
+from lldbsuite.test import lldbutil
+
+
+class LibcxxStringDataFormatterSimulatorTestCase(TestBase):
+
+    mydir = TestBase.compute_mydir(__file__)
+    NO_DEBUG_INFO_TESTCASE = True
+
+    def _run_test(self, defines):
+        cxxflags_extras = " ".join(["-D%s" % d for d in defines])
+        self.build(dictionary=dict(CXXFLAGS_EXTRAS=cxxflags_extras))
+        lldbutil.run_to_source_breakpoint(self, '// Break here',
+                lldb.SBFileSpec("main.cpp"))
+        self.expect_var_path("shortstring", summary='"short"')
+        self.expect_var_path("longstring", summary='"I am a very long string"')
+
+    def test_v1_layout(self):
+        """ Current v1 layout. """
+        self._run_test([])
+
+    def test_v2_layout(self):
+        """ Current v2 layout. """
+        self._run_test(["ALTERNATE_LAYOUT"])
+
+    def test_v1_layout_bitmasks(self):
+        """ Pre-D123580 v1 layout. """
+        self._run_test(["BITMASKS"])
+
+    def test_v2_layout_bitmasks(self):
+        """ Pre-D123580 v2 layout. """
+        self._run_test(["ALTERNATE_LAYOUT", "BITMASKS"])
+
+    def test_v2_layout_subclass_padding(self):
+        """ Pre-c3d0205ee771 v2 layout. """
+        self._run_test(["ALTERNATE_LAYOUT", "BITMASKS", "SUBCLASS_PADDING"])
+
diff --git a/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx/string/simulator/main.cpp b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx/string/simulator/main.cpp
new file mode 100644
index 0000000000000..4852dfd456687
--- /dev/null
+++ b/lldb/test/API/functionalities/data-formatter/data-formatter-stl/libcxx/string/simulator/main.cpp
@@ -0,0 +1,217 @@
+#include <climits>
+#include <memory>
+#include <type_traits>
+
+namespace std {
+namespace __lldb {
+
+template <class _Tp, int _Idx,
+          bool _CanBeEmptyBase =
+              std::is_empty<_Tp>::value && !std::is_final<_Tp>::value>
+struct __compressed_pair_elem {
+  explicit __compressed_pair_elem(_Tp __t) : __value_(__t) {}
+
+  _Tp &__get() { return __value_; }
+
+private:
+  _Tp __value_;
+};
+
+template <class _Tp, int _Idx>
+struct __compressed_pair_elem<_Tp, _Idx, true> : private _Tp {
+  explicit __compressed_pair_elem(_Tp __t) : _Tp(__t) {}
+
+  _Tp &__get() { return *this; }
+};
+
+template <class _T1, class _T2>
+class __compressed_pair : private __compressed_pair_elem<_T1, 0>,
+                          private __compressed_pair_elem<_T2, 1> {
+public:
+  using _Base1 = __compressed_pair_elem<_T1, 0>;
+  using _Base2 = __compressed_pair_elem<_T2, 1>;
+
+  explicit __compressed_pair(_T1 __t1, _T2 __t2) : _Base1(__t1), _Base2(__t2) {}
+
+  _T1 &first() { return static_cast<_Base1 &>(*this).__get(); }
+};
+
+#if defined(ALTERNATE_LAYOUT) && defined(SUBCLASS_PADDING)
+template <class _CharT, size_t = sizeof(_CharT)> struct __padding {
+  unsigned char __xx[sizeof(_CharT) - 1];
+};
+
+template <class _CharT> struct __padding<_CharT, 1> {};
+#endif
+
+template <class _CharT, class _Traits, class _Allocator> class basic_string {
+public:
+  typedef _CharT value_type;
+  typedef _Allocator allocator_type;
+  typedef allocator_traits<allocator_type> __alloc_traits;
+  typedef typename __alloc_traits::size_type size_type;
+  typedef typename __alloc_traits::pointer pointer;
+
+#ifdef ALTERNATE_LAYOUT
+
+  struct __long {
+    pointer __data_;
+    size_type __size_;
+#ifdef BITMASKS
+    size_type __cap_;
+#else
+    size_type __cap_ : sizeof(size_type) * CHAR_BIT - 1;
+    size_type __is_long_ : 1;
+#endif
+  };
+
+  enum {
+    __min_cap = (sizeof(__long) - 1) / sizeof(value_type) > 2
+                    ? (sizeof(__long) - 1) / sizeof(value_type)
+                    : 2
+  };
+
+  struct __short {
+    value_type __data_[__min_cap];
+#ifdef BITMASKS
+#ifdef SUBCLASS_PADDING
+    struct : __padding<value_type> {
+      unsigned char __size_;
+    };
+#else
+    unsigned char __padding[sizeof(value_type) - 1];
+    unsigned char __size_;
+#endif
+#else // !BITMASKS
+    unsigned char __size_ : 7;
+    unsigned char __is_long_ : 1;
+#endif
+  };
+
+#ifdef BITMASKS
+#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+  static const size_type __short_shift = 1;
+  static const size_type __long_mask = 0x1ul;
+#else
+  static const size_type __short_shift = 0;
+  static const size_type __long_mask = ~(size_type(~0) >> 1);
+#endif
+#else // !BITMASKS
+#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+  static const size_type __endian_factor = 2;
+#else
+  static const size_type __endian_factor = 1;
+#endif
+#endif // BITMASKS
+
+#else // !ALTERNATE_LAYOUT
+
+  struct __long {
+#ifdef BITMASKS
+    size_type __cap_;
+#else
+    size_type __is_long_ : 1;
+    size_type __cap_ : sizeof(size_type) * CHAR_BIT - 1;
+#endif
+    size_type __size_;
+    pointer __data_;
+  };
+
+  enum {
+    __min_cap = (sizeof(__long) - 1) / sizeof(value_type) > 2
+                    ? (sizeof(__long) - 1) / sizeof(value_type)
+                    : 2
+  };
+
+  struct __short {
+    union {
+#ifdef BITMASKS
+      unsigned char __size_;
+#else
+      struct {
+        unsigned char __is_long_ : 1;
+        unsigned char __size_ : 7;
+      };
+#endif
+      value_type __lx;
+    };
+    value_type __data_[__min_cap];
+  };
+
+#ifdef BITMASKS
+#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+  static const size_type __short_shift = 0;
+  static const size_type __long_mask = ~(size_type(~0) >> 1);
+#else
+  static const size_type __short_shift = 1;
+  static const size_type __long_mask = 0x1ul;
+#endif
+#else // !BITMASKS
+#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+  static const size_type __endian_factor = 1;
+#else
+  static const size_type __endian_factor = 2;
+#endif
+#endif
+
+#endif // ALTERNATE_LAYOUT
+
+  union __ulx {
+    __long __lx;
+    __short __lxx;
+  };
+
+  enum { __n_words = sizeof(__ulx) / sizeof(size_type) };
+
+  struct __raw {
+    size_type __words[__n_words];
+  };
+
+  struct __rep {
+    union {
+      __long __l;
+      __short __s;
+      __raw __r;
+    };
+  };
+
+  __compressed_pair<__rep, allocator_type> __r_;
+
+public:
+  template <size_t __N>
+  basic_string(unsigned char __size, const value_type (&__data)[__N])
+      : __r_({}, {}) {
+    static_assert(__N < __min_cap, "");
+#ifdef BITMASKS
+    __r_.first().__s.__size_ = __size << __short_shift;
+#else
+    __r_.first().__s.__size_ = __size;
+    __r_.first().__s.__is_long_ = false;
+#endif
+    for (size_t __i = 0; __i < __N; ++__i)
+      __r_.first().__s.__data_[__i] = __data[__i];
+  }
+  basic_string(size_t __cap, size_type __size, pointer __data) : __r_({}, {}) {
+#ifdef BITMASKS
+    __r_.first().__l.__cap_ = __cap | __long_mask;
+#else
+    __r_.first().__l.__cap_ = __cap / __endian_factor;
+    __r_.first().__l.__is_long_ = true;
+#endif
+    __r_.first().__l.__size_ = __size;
+    __r_.first().__l.__data_ = __data;
+  }
+};
+
+using string = basic_string<char, std::char_traits<char>, std::allocator<char>>;
+
+} // namespace __lldb
+} // namespace std
+
+int main() {
+  char longdata[] = "I am a very long string";
+  std::__lldb::string longstring(sizeof(longdata), sizeof(longdata) - 1,
+                                 longdata);
+  std::__lldb::string shortstring(5, "short");
+  return 0; // Break here
+}