diff --git a/libcxx/test/libcxx/strings/basic.string/alignof.compile.pass.cpp b/libcxx/test/libcxx/strings/basic.string/alignof.compile.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/strings/basic.string/alignof.compile.pass.cpp
@@ -0,0 +1,136 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// Ensure that we never change the size or alignment of `basic_string`
+
+// UNSUPPORTED: c++03
+
+#include <string>
+
+#include "test_macros.h"
+#include "min_allocator.h"
+#include "test_allocator.h"
+
+template <class T>
+class small_pointer {
+  uint16_t offset;
+};
+
+template <class T>
+class small_iter_allocator {
+public:
+  using value_type = T;
+  using pointer = small_pointer<T>;
+  using size_type = int16_t;
+  using difference_type = int16_t;
+
+  small_iter_allocator() TEST_NOEXCEPT {}
+
+  template <class U>
+  small_iter_allocator(small_iter_allocator<U>) TEST_NOEXCEPT {}
+
+  T* allocate(std::size_t n);
+  void deallocate(T* p, std::size_t);
+
+  friend bool operator==(small_iter_allocator, small_iter_allocator) { return true; }
+  friend bool operator!=(small_iter_allocator, small_iter_allocator) { return false; }
+};
+
+template <class CharT>
+using min_string = std::basic_string<CharT, std::char_traits<CharT>, min_allocator<CharT>>;
+
+template <class CharT>
+using test_string = std::basic_string<CharT, std::char_traits<CharT>, test_allocator<CharT>>;
+
+template <class CharT>
+using small_string = std::basic_string<CharT, std::char_traits<CharT>, small_iter_allocator<CharT>>;
+
+#if __SIZE_WIDTH__ == 64
+
+static_assert(alignof(std::string) == 8, "");
+static_assert(alignof(min_string<char>) == 8, "");
+static_assert(alignof(test_string<char>) == 8, "");
+static_assert(alignof(small_string<char>) == 2, "");
+
+#  ifndef TEST_HAS_NO_WIDE_CHARACTERS
+#    if __WCHAR_WIDTH__ == 32
+static_assert(alignof(std::wstring) == 8, "");
+static_assert(alignof(min_string<wchar_t>) == 8, "");
+static_assert(alignof(test_string<wchar_t>) == 8, "");
+static_assert(alignof(small_string<wchar_t>) == 4, "");
+#    elif __WCHAR_WIDTH__ == 16
+static_assert(alignof(std::wstring) == 8, "");
+static_assert(alignof(min_string<wchar_t>) == 8, "");
+static_assert(alignof(test_string<wchar_t>) == 8, "");
+static_assert(alignof(small_string<wchar_t>) == 2, "");
+#    else
+#      error "Unexpected wchar_t width"
+#    endif
+#  endif
+
+#  ifndef TEST_HAS_NO_CHAR8_T
+static_assert(alignof(std::u8string) == 8, "");
+static_assert(alignof(min_string<char8_t>) == 8, "");
+static_assert(alignof(test_string<char8_t>) == 8, "");
+static_assert(alignof(small_string<char8_t>) == 2, "");
+#  endif
+
+#  ifndef TEST_HAS_NO_UNICODE_CHARS
+static_assert(alignof(std::u16string) == 8, "");
+static_assert(alignof(std::u32string) == 8, "");
+static_assert(alignof(min_string<char16_t>) == 8, "");
+static_assert(alignof(min_string<char32_t>) == 8, "");
+static_assert(alignof(test_string<char16_t>) == 8, "");
+static_assert(alignof(test_string<char32_t>) == 8, "");
+static_assert(alignof(small_string<char16_t>) == 2, "");
+static_assert(alignof(small_string<char32_t>) == 4, "");
+#  endif
+
+#elif __SIZE_WIDTH__ == 32
+
+static_assert(alignof(std::string) == 4, "");
+static_assert(alignof(min_string<char>) == 4, "");
+static_assert(alignof(test_string<char>) == 4, "");
+static_assert(alignof(small_string<char>) == 2, "");
+
+#  ifndef TEST_HAS_NO_WIDE_CHARACTERS
+#    if __WCHAR_WIDTH__ == 32
+static_assert(alignof(std::wstring) == 4, "");
+static_assert(alignof(min_string<wchar_t>) == 4, "");
+static_assert(alignof(test_string<wchar_t>) == 4, "");
+static_assert(alignof(small_string<wchar_t>) == 4, "");
+#    elif __WCHAR_WIDTH__ == 16
+static_assert(alignof(std::wstring) == 4, "");
+static_assert(alignof(min_string<wchar_t>) == 4, "");
+static_assert(alignof(test_string<wchar_t>) == 4, "");
+static_assert(alignof(small_string<wchar_t>) == 2, "");
+#    else
+#      error "Unexpected wchar_t width"
+#    endif
+#  endif
+
+#  ifndef TEST_HAS_NO_CHAR8_T
+static_assert(alignof(std::u8string) == 4, "");
+static_assert(alignof(min_string<char8_t>) == 4, "");
+static_assert(alignof(test_string<char8_t>) == 4, "");
+static_assert(alignof(small_string<char8_t>) == 2, "");
+#  endif
+
+#  ifndef TEST_HAS_NO_UNICODE_CHARS
+static_assert(alignof(std::u16string) == 4, "");
+static_assert(alignof(std::u32string) == 4, "");
+static_assert(alignof(min_string<char16_t>) == 4, "");
+static_assert(alignof(min_string<char32_t>) == 4, "");
+static_assert(alignof(test_string<char16_t>) == 4, "");
+static_assert(alignof(test_string<char32_t>) == 4, "");
+static_assert(alignof(small_string<char32_t>) == 4, "");
+#  endif
+
+#else
+#  error "size_t has an unexpected size"
+#endif
diff --git a/libcxx/test/libcxx/strings/basic.string/sizeof.compile.pass.cpp b/libcxx/test/libcxx/strings/basic.string/sizeof.compile.pass.cpp
new file mode 100644
--- /dev/null
+++ b/libcxx/test/libcxx/strings/basic.string/sizeof.compile.pass.cpp
@@ -0,0 +1,135 @@
+//===----------------------------------------------------------------------===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+// Ensure that we never change the size or alignment of `basic_string`
+
+#include <string>
+
+#include "test_macros.h"
+#include "min_allocator.h"
+#include "test_allocator.h"
+
+template <class T>
+class small_pointer {
+  uint16_t offset;
+};
+
+template <class T>
+class small_iter_allocator {
+public:
+  using value_type = T;
+  using pointer = small_pointer<T>;
+  using size_type = int16_t;
+  using difference_type = int16_t;
+
+  small_iter_allocator() TEST_NOEXCEPT {}
+
+  template <class U>
+  small_iter_allocator(small_iter_allocator<U>) TEST_NOEXCEPT {}
+
+  T* allocate(std::size_t n);
+  void deallocate(T* p, std::size_t);
+
+  friend bool operator==(small_iter_allocator, small_iter_allocator) { return true; }
+  friend bool operator!=(small_iter_allocator, small_iter_allocator) { return false; }
+};
+
+template <class CharT>
+using min_string = std::basic_string<CharT, std::char_traits<CharT>, min_allocator<CharT> >;
+
+template <class CharT>
+using test_string = std::basic_string<CharT, std::char_traits<CharT>, test_allocator<CharT> >;
+
+template <class CharT>
+using small_string = std::basic_string<CharT, std::char_traits<CharT>, small_iter_allocator<CharT> >;
+
+#if __SIZE_WIDTH__ == 64
+
+static_assert(sizeof(std::string) == 24, "");
+static_assert(sizeof(min_string<char>) == 24, "");
+static_assert(sizeof(test_string<char>) == 32, "");
+static_assert(sizeof(small_string<char>) == 6, "");
+
+#  ifndef TEST_HAS_NO_WIDE_CHARACTERS
+#    if __WCHAR_WIDTH__ == 32
+static_assert(sizeof(std::wstring) == 24, "");
+static_assert(sizeof(min_string<wchar_t>) == 24, "");
+static_assert(sizeof(test_string<wchar_t>) == 32, "");
+static_assert(sizeof(small_string<wchar_t>) == 12, "");
+#    elif __WCHAR_WIDTH__ == 16
+static_assert(sizeof(std::wstring) == 24, "");
+static_assert(sizeof(min_string<wchar_t>) == 24, "");
+static_assert(sizeof(test_string<wchar_t>) == 32, "");
+static_assert(sizeof(small_string<wchar_t>) == 6, "");
+#    else
+#      error "Unexpected wchar_t width"
+#    endif
+#  endif
+
+#  ifndef TEST_HAS_NO_CHAR8_T
+static_assert(sizeof(std::u8string) == 24, "");
+static_assert(sizeof(min_string<char8_t>) == 24, "");
+static_assert(sizeof(test_string<char8_t>) == 32, "");
+static_assert(sizeof(small_string<char8_t>) == 6, "");
+#  endif
+
+#  ifndef TEST_HAS_NO_UNICODE_CHARS
+static_assert(sizeof(std::u16string) == 24, "");
+static_assert(sizeof(std::u32string) == 24, "");
+static_assert(sizeof(min_string<char16_t>) == 24, "");
+static_assert(sizeof(min_string<char32_t>) == 24, "");
+static_assert(sizeof(test_string<char16_t>) == 32, "");
+static_assert(sizeof(test_string<char32_t>) == 32, "");
+static_assert(sizeof(small_string<char16_t>) == 6, "");
+static_assert(sizeof(small_string<char32_t>) == 12, "");
+#  endif
+
+#elif __SIZE_WIDTH__ == 32
+
+static_assert(sizeof(std::string) == 12, "");
+static_assert(sizeof(min_string<char>) == 12, "");
+static_assert(sizeof(test_string<char>) == 24, "");
+static_assert(sizeof(small_string<char>) == 6, "");
+
+#  ifndef TEST_HAS_NO_WIDE_CHARACTERS
+#    if __WCHAR_WIDTH__ == 32
+static_assert(sizeof(std::wstring) == 12, "");
+static_assert(sizeof(min_string<wchar_t>) == 12, "");
+static_assert(sizeof(test_string<wchar_t>) == 24, "");
+static_assert(sizeof(small_string<wchar_t>) == 12, "");
+#    elif __WCHAR_WIDTH__ == 16
+static_assert(sizeof(std::wstring) == 12, "");
+static_assert(sizeof(min_string<wchar_t>) == 12, "");
+static_assert(sizeof(test_string<wchar_t>) == 24, "");
+static_assert(sizeof(small_string<wchar_t>) == 6, "");
+#    else
+#      error "Unexpected wchar_t width"
+#    endif
+#  endif
+
+#  ifndef TEST_HAS_NO_CHAR8_T
+static_assert(sizeof(std::u8string) == 12, "");
+static_assert(sizeof(min_string<char8_t>) == 12, "");
+static_assert(sizeof(test_string<char8_t>) == 24, "");
+static_assert(sizeof(small_string<char>) == 6, "");
+#  endif
+
+#  ifndef TEST_HAS_NO_UNICODE_CHARS
+static_assert(sizeof(std::u16string) == 12, "");
+static_assert(sizeof(std::u32string) == 12, "");
+static_assert(sizeof(min_string<char16_t>) == 12, "");
+static_assert(sizeof(min_string<char32_t>) == 12, "");
+static_assert(sizeof(test_string<char16_t>) == 24, "");
+static_assert(sizeof(test_string<char32_t>) == 24, "");
+static_assert(sizeof(small_string<char16_t>) == 6, "");
+static_assert(sizeof(small_string<char32_t>) == 12, "");
+#  endif
+
+#else
+#  error "size_t has an unexpected size"
+#endif