[libc++] Add missed comment in https://reviews.llvm.org/D113910

diff --git a/libcxx/include/string b/libcxx/include/string
index 5d8473b..6114220 100644 (file)
--- a/libcxx/include/string
+++ b/libcxx/include/string
@@ -2658,7 +2658,7 @@ basic_string<_CharT, _Traits, _Allocator>::push_back(value_type __c)
     if (__sz == __cap)
     {
         __grow_by(__cap, 1, __sz, __sz, 0);
-        __is_short = false;
+        __is_short = false; // the string is always long after __grow_by
     }
     pointer __p;
     if (__is_short)