https://gcc.gnu.org/bugzilla/show_bug.cgi?id=127918
Bug ID: 127918
Summary: ranges::mismatch fails to use borrowed_iterator_t
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libstdc++
Assignee: unassigned at gcc dot gnu.org
Reporter: jesseconnell at gmail dot com
Target Milestone: ---
Created attachment 65858
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=65858&action=edit
Patch with proposed fix
The range overload of std::ranges::mismatch uses iterator_t instead of
borrowed_iterator_t for both result members, so non-borrowed rvalue ranges
produce iterators instead of ranges::dangling.
#include <algorithm>
#include <concepts>
#include <ranges>
#include <vector>
int main()
{
using namespace std;
vector<int> v{1, 2, 3};
auto a = ranges::mismatch(vector<int>{1, 2, 3}, v);
static_assert(same_as<decltype(a.in1), ranges::dangling>);
static_assert(same_as<decltype(a.in2), vector<int>::iterator>);
auto b = ranges::mismatch(v, vector<int>{1, 2, 3});
static_assert(same_as<decltype(b.in1), vector<int>::iterator>);
static_assert(same_as<decltype(b.in2), ranges::dangling>);
auto c = ranges::mismatch(vector<int>{1, 2, 3}, vector<int>{1, 2, 3});
static_assert(same_as<decltype(c.in1), ranges::dangling>);
static_assert(same_as<decltype(c.in2), ranges::dangling>);
}
Command:
g++-16 -std=c++20 -Wall -Wextra -c mismatch.cc
Expected: compilation succeeds.
Actual: all assertions expecting ranges::dangling fail.
Specified return type:
https://eel.is/c++draft/alg.mismatch
Reproduced with GCC 16.0.1 20260315 (experimental), trunk
r16-8100-g3aca3bae8ee, Ubuntu package 16-20260315-1ubuntu1~22~ppa1, targeting
x86_64-linux-gnu.
A proposed fix with regression tests is attached.
