[Bug libstdc++/71950] New: std::ios_base::failure.what() returns irrelevant error message

["sytelus at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=71950

            Bug ID: 71950
           Summary: std::ios_base::failure.what() returns irrelevant error
                    message
           Product: gcc
           Version: 5.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: sytelus at gmail dot com
  Target Milestone: ---

The what() method of exception class std::ios_base::failure should return
helpful error messages indicating why IO operation failed. At present it
returns irrelevant and unhelpful message such as "basic_ios::clear".

Below is the code to reproduce this bug:

#include <iostream>
#include <fstream>

int main() {
    try {
        std::ofstream flog;
        flog.exceptions(flog.exceptions() | std::ios::failbit |
std::ifstream::badbit);
        flog.open("~/bad/path/xyz.tsv", std::ios::trunc);
    }
    catch (const std::ios_base::failure &ex) {    
        std::cout << ex.what();
    }
}