diff --git a/libcxx/include/uchar.h b/libcxx/include/uchar.h
--- a/libcxx/include/uchar.h
+++ b/libcxx/include/uchar.h
@@ -41,12 +41,23 @@
 #if !defined(_LIBCPP_CXX03_LANG)
 
 // Some platforms don't implement <uchar.h> and we don't want to give a hard
-// error on those platforms. When the platform doesn't provide <uchar.h>, at
-// least include <stddef.h> so we get the declaration for size_t.
+// error on those platforms.
 #if __has_include_next(<uchar.h>)
 # include_next <uchar.h>
 #else
+// When the platform doesn't provide <uchar.h>, try to get the declaration
+// for mbstate_t.
+# if __has_include(<bits/types/mbstate_t.h>)
+#  include <bits/types/mbstate_t.h> // works on most Unixes
+# elif __has_include(<sys/_types/_mbstate_t.h>)
+#  include <sys/_types/_mbstate_t.h> // works on Darwin
+# elif !defined(_LIBCPP_HAS_NO_WIDE_CHARACTERS)
+#  include_next <wchar.h>
+#endif
+
+// Get the declaration for size_t.
 # include <stddef.h>
+
 #endif
 
 #endif // _LIBCPP_CXX03_LANG