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From: "richardpku at gmail dot com" <gcc-bugzilla@gcc.gnu.org>
To: gcc-bugs@gcc.gnu.org
Subject: [Bug libstdc++/99181] New: char_traits<char> (and thus string_view)
 compares strings differently in constexpr and non-constexpr contexts
Date: Sat, 20 Feb 2021 15:47:57 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D99181

            Bug ID: 99181
           Summary: char_traits<char> (and thus string_view) compares
                    strings differently in constexpr and non-constexpr
                    contexts
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: libstdc++
          Assignee: unassigned at gcc dot gnu.org
          Reporter: richardpku at gmail dot com
  Target Milestone: ---

Minimal program to produce bug (run it on a platform where char is a signed
type, such as i386/x86-64):

/tmp % cat a.cpp
#include <string_view>
#include <iostream>

using namespace std;

int main() {
        // constexpr
        constexpr bool i =3D ("\xff"sv > "aaa"sv);
        cout << i << ",";

        // not constexpr
        auto a =3D "\xff"sv, b =3D "aaa"sv;
        cout << (a > b) << endl;

        return 0;
}
/tmp % g++ -std=3Dgnu++2a a.cpp && ./a.out
0,1

The expected result is "1,1".


In a non-constexpr context, std::char_traits<char>::compare invokes
__builtin_memcmp, which is required by C standard to interpret characters as
unsigned char.

In a constexpr context, however, std::char_traits<char>::compare invokes
__gnu_cxx::char_traits<char>::compare, which in turn calls
__gnu_cxx::char_traits<char>::lt to compare chars.
__gnu_cxx::char_traits<char>::lt (unlike std::char_traits<char>::lt) is not
specialized to compare chars as unsigned char.=