https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99181
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 = ("\xff"sv > "aaa"sv);
cout << i << ",";
// not constexpr
auto a = "\xff"sv, b = "aaa"sv;
cout << (a > b) << endl;
return 0;
}
/tmp % g++ -std=gnu++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.
