commit fb061814aa724727c5761b53e5a60007355da210
Author: Jonathan Wakely <jwakely@redhat.com>
Date:   Thu Feb 27 11:20:54 2020

    libstdc++: Detect null __normal_iterator in valid range checks
    
    Enahnce debug mode checks for valid ranges to detect value-initialized
    __normal_iterator objects, so that basic_string iterators can be
    checked.
    
    Signed-off-by: Jonathan Wakely <jwakely@redhat.com>
    
    libstdc++-v3/ChangeLog:
    
            * include/debug/helper_functions.h (__check_singular): Add overload
            for __normal_iterator that checks for value-initialized iterators.

diff --git a/libstdc++-v3/include/debug/helper_functions.h b/libstdc++-v3/include/debug/helper_functions.h
index c0144ced979..e0c69151e3a 100644
--- a/libstdc++-v3/include/debug/helper_functions.h
+++ b/libstdc++-v3/include/debug/helper_functions.h
@@ -36,6 +36,13 @@
 
 #include <bits/stl_pair.h>			// for pair
 
+namespace __gnu_cxx
+{
+_GLIBCXX_BEGIN_NAMESPACE_VERSION
+  template<typename _Iterator, typename _Container> class __normal_iterator;
+_GLIBCXX_END_NAMESPACE_VERSION
+}
+
 namespace __gnu_debug
 {
   template<typename _Iterator, typename _Sequence, typename _Category>
@@ -144,7 +151,13 @@ namespace __gnu_debug
 	__ptr == 0;
     }
 
-  /** We say that integral types for a valid range, and defer to other
+  template<typename _Tp, typename _Cont>
+    _GLIBCXX_CONSTEXPR
+    inline bool
+    __check_singular(__gnu_cxx::__normal_iterator<_Tp*, _Cont> const& __x)
+    { return __x.base() == 0; }
+
+  /** We say that integral types form a valid range, and defer to other
    *  routines to realize what to do with integral types instead of
    *  iterators.
   */