[Bug libstdc++/79254] [5/6/7 Regression] basic_string::operator= isn't exception safe

["redi at gcc dot gnu.org" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=79254

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
      Known to work|                            |4.9.4
            Version|unknown                     |5.4.1
            Summary|basic_string::operator=     |[5/6/7 Regression]
                   |isn't exception safe        |basic_string::operator=
                   |                            |isn't exception safe
      Known to fail|                            |5.4.0, 6.3.0, 7.0

--- Comment #4 from Jonathan Wakely <redi at gcc dot gnu.org> ---
If we use a C++03-compatible allocator this variation on the testcase shows
it's a regression compared to the old COW string, which didn't have this
problem (partly because it doesn't support allocator propagation):

#include <string>
#include <stdio.h>

int counter = 0;

template <typename T>
struct alloc : std::allocator<T>
{
    using value_type                             = T;
    using propagate_on_container_copy_assignment = std::true_type;
    template<typename U>
      struct rebind { typedef alloc<U> other; };

    alloc(int id = 0)
    : id(id)
    {
    }

    template <typename U>
    alloc(const alloc<U>& other) : id(other.id) { }

    T* allocate(std::size_t n)
    {
      if (++counter == 3)
        throw 1;
      return std::allocator<T>::allocate(n);
    }

    int id;
};

template <typename T, typename U>
bool operator==(const alloc<T>& lhs, const alloc<U>& rhs)
{
    return lhs.id == rhs.id;
}

template <typename T, typename U>
bool operator!=(const alloc<T>& lhs, const alloc<U>& rhs)
{
    return lhs.id != rhs.id;
}

int main()
{
    using broken_string = std::basic_string<char,
                                            std::char_traits<char>,
                                            alloc<char>>;

    broken_string s1("This is my first string",  alloc<char>(1));
    broken_string s2("This is my second string", alloc<char>(2));
    s2.c_str();  // Prevent the string being shared, forcing a copy.

    try {
        s1 = s2;
    }
    catch (int) {
        puts("Caught exception.");
    }

    printf("s1 = %s\n", s1.c_str());
    printf("s2 = %s\n", s2.c_str());
}