We have been talking for a long time of a debug mode with less impact on 
performances.

I propose to simply use the existing _GLIBCXX_ASSERTIONS macro.

     libstdc++: [_GLIBCXX_ASSERTIONS] Activate basic debug checks

     Use _GLIBCXX_ASSERTIONS as a _GLIBCXX_DEBUG light mode. When 
defined it activates
     all _GLIBCXX_DEBUG checks but skipping those requiring to loop 
through the iterator
     range unless in case of constexpr.

     libstdc++-v3/ChangeLog:

             * include/debug/debug.h [_GLIBCXX_ASSERTIONS]: Define debug 
macros non-empty.
             * include/debug/helper_functions.h: Cleanup comment about 
removed _Iter_base.
             * include/debug/functions.h (__skip_debug_runtime_check): 
New, returns false if
             _GLIBCXX_DEBUG is defined or if constant evaluated.
             (__check_sorted, __check_partitioned_lower, 
__check_partitioned_upper): Use latter.

Tested under Linux x64.

Ok to commit ?

François