[Patch, libstdc++/63497] Avoid dereferencing invalid iterator in regex_executor

[Jonathan Wakely]

On 20/10/14 10:23 -0700, Tim Shen wrote:
>Bootstrapped and tested.

Did you manage to produce a testcase that crashed on trunk?

>@@ -407,25 +409,28 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
>   template<typename _BiIter, typename _Alloc, typename _TraitsT,
> 	   bool __dfs_mode>
>     bool _Executor<_BiIter, _Alloc, _TraitsT, __dfs_mode>::
>-    _M_word_boundary(_State<_TraitsT>) const
>+    _M_word_boundary(_State<_TraitsT>)
>     {
>-      // By definition.
>-      bool __ans = false;
>-      auto __pre = _M_current;
>-      --__pre;
>-      if (!(_M_at_begin() && _M_at_end()))
>+      bool __left_is_word = false;
>+      if (_M_current != _M_begin
>+	  || (_M_flags & regex_constants::match_prev_avail))
> 	{
>-	  if (_M_at_begin())
>-	    __ans = _M_is_word(*_M_current)
>-	      && !(_M_flags & regex_constants::match_not_bow);
>-	  else if (_M_at_end())
>-	    __ans = _M_is_word(*__pre)
>-	      && !(_M_flags & regex_constants::match_not_eow);
>-	  else
>-	    __ans = _M_is_word(*_M_current)
>-	      != _M_is_word(*__pre);
>+	  --_M_current;
>+	  if (_M_is_word(*_M_current))
>+	    __left_is_word = true;
>+	  ++_M_current;

Is it really necessary to modify _M_current here?
Couldn't you do:

       auto __pre = _M_current;
       if (_M_is_word(*--__pre))
         __left_is_word = true;

Then the function could remain const, couldn't it?