[Bug c++/66170] Bogus warning with -Wsign-conversion when using static_cast<size_t> on an int

["sir.vestnik at gmail dot com" <gcc-bugzilla at gcc dot gnu dot org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66170

Sergey Vidyuk <sir.vestnik at gmail dot com> changed:

           What    |Removed                     |Added
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                 CC|                            |sir.vestnik at gmail dot com

--- Comment #1 from Sergey Vidyuk <sir.vestnik at gmail dot com> ---
I have similair issue with gcc-6.2

[vestnik@VestniK-laptop C++]$ cat fasttest.cpp 
#include <iostream>
#include <vector>

void foo(const std::vector<int> vec, const int idx) {
        if (idx < 0 || vec.size() <= static_cast<size_t>(idx))
        {
                std::cout << "No such index\n";
                return;
        }
        std::cout << "Valid index\n";
}

int main() {
    foo({1, 2, 3, 4 ,5}, 6);
    return 0;
}
[vestnik@VestniK-laptop C++]$ g++ -std=c++11 -Wsign-conversion -Wall -Werror
fasttest.cpp -o fasttest
fasttest.cpp: In function 'void foo(std::vector<int>, int)':
fasttest.cpp:5:31: error: conversion to 'std::vector<int>::size_type {aka long
unsigned int}' from 'const int' may change the sign of the result
[-Werror=sign-conversion]
  if (idx < 0 || vec.size() <= static_cast<size_t>(idx))
                               ^~~~~~~~~~~~~~~~~~~~~~~~
cc1plus: all warnings being treated as errors
[vestnik@VestniK-laptop C++]$ g++ --version
g++ (GCC) 6.2.1 20160830
Copyright (C) 2016 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.


It looks like static_cast type was incorrectly treated as const int instead of
size_t. Changing static_cast<size_t>(idx) to static_cast<unsigned>(idx)
silences diagnostics but such fix looks lihe a strange hack.