[GCC] C++ program calling std::getline() crashes with certain compilation flags

[TANNHAUSER Falk]

The following C++ program crashes when compiled with GCC (both 5.4 and 6.3) under Cygwin, when compiled with both an optimization level higher than -O0 (i.e. -O1, -O2 or -O3) and the C++ standard set to -std=c++nn (for any supported nn, i.e. 98, 03, 11, 14 or 17):
```
#include <iostream>
#include <string>
int main()
{
    std::string s;
    std::getline(std::cin, s);
    std::cout << "You entered \"" << s << "\".\n";
    return 0;
}
```
On the other hand, when compiled with -std=gnu++nn or -O0, the program executes normally. For details, run the attached Bash script test_getline.sh, which produces the output contained in the attached file test_getline_Cygwin.txt . No similar problem shows with GCC under Linux.
It looks similar to the problem reported in https://cygwin.com/ml/cygwin/2017-07/msg00088.html which appears to be circumvented by passing -std=gnu++11 instead of -std=c++11.
Under GDB, a faulty executable produces a message like:
```
Thread 1 "test_getline" received signal SIGABRT, Aborted.
0x00000003bf12576b in cygstdc++-6!_ZNSs7reserveEm () from /usr/bin/cygstdc++-6.dll
```
Un-mangling _ZNSs7reserveEm through c++filt gives
std::basic_string<char, std::char_traits<char>, std::allocator<char> >::reserve(unsigned long)

Falk
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