piecewise-construct-ub

README.md

$ g++ --version
g++ (GCC) 12.1.0
$ g++ -O2 -std=c++14 a.cpp main.cpp && ./a.out
a.out: main.cpp:5: int main(): Assertion `f() == (*c().target<F*>())()' failed.
Aborted

$ clang++ --version
clang version 14.0.5
$ clang++ -O2 -std=c++14 a.cpp main.cpp && ./a.out
a.out: main.cpp:5: int main(): Assertion `f() == (*c().target<F*>())()' failed.
Aborted

a.cpp

#include "odr.hpp"
std::function<F> c() { return f; }

main.cpp

#include "odr.hpp"
#include <cassert>
int main() {
    assert(f == *c().target<F*>()); // passes
    assert(f() == (*c().target<F*>())()); // fails
}

odr.hpp

#include <utility>
#include <functional>
inline void const* f() { return &std::piecewise_construct; }
using F = void const*();
std::function<F> c();