Hi

Here is the patch to add backtrace to _GLIBCXX_DEBUG mode. It is using 
the recently introduced _GLIBCXX_HAS_STACKTRACE.

It is only WIP because for the moment I had to add:

// { dg-options "-lstdc++_libbacktrace" }

to avoid unresolved symbols.

How should I handle this ?

1. Ask the user to define _GLIBCXX_DEBUG_STACKTRACE to get the backtrace 
and so also require to add this linker option.

2. Is it possible to add something in formatter.h to automagically link 
libstdc++_libbacktrace ?

3. Add symbols to libstdc++.so ?

Output sample:

/home/fdt/dev/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/debug/vector:264:
In function:
     void std::debug::vector<_Tp, _Allocator>::assign(_InputIterator,
     _InputIterator) [with _InputIterator = const int*;
     <template-parameter-2-2> = void; _Tp = int; _Allocator =
     std::allocator<int>]

Backtrace:
     0x40284a void std::debug::vector<int, std::allocator<int> 
 >::assign<int const*, void>(int const*, int const*)
/home/fdt/dev/gcc/build/x86_64-pc-linux-gnu/libstdc++-v3/include/debug/vector:264
     0x402bed void gnu_test::check_assign1<std::debug::vector<int, 
std::allocator<int> > >()
/home/fdt/dev/gcc/git/libstdc++-v3/testsuite/util/debug/checks.h:111
     0x40246c test01()
/home/fdt/dev/gcc/git/libstdc++-v3/testsuite/23_containers/vector/debug/assign4_neg.cc:26
     0x40246c main
/home/fdt/dev/gcc/git/libstdc++-v3/testsuite/23_containers/vector/debug/assign4_neg.cc:31

Error: function requires a valid iterator range [first, last).

Objects involved in the operation:
     iterator "first" @ 0x7ffe09e62fd8 {
       type = int const* (constant iterator);
     }
     iterator "last" @ 0x7ffe09e62fd0 {
       type = int const* (constant iterator);
     }
XFAIL: 23_containers/vector/debug/assign4_neg.cc execution test

François