reference_wrapper doesn't handle function pointers with C-style varargs

[mclow]

Bugzilla Link	35664
Version	unspecified
OS	All
CC	@mclow,@jwakely

Extended Description

The following code should compile, but doesn't

#include <functional>

int f(const char*, ...) { return 0; }
std::reference_wrapper fr{f};
decltype(fr)::result_type i;

struct X {
    int f(const char*, ...) { return 0; }
};
auto xf = &X::f;
std::reference_wrapper xfr{xf};
decltype(xfr)::result_type j;

int main() {}

Reported by Jonathan Wakeley

[mclow]

assigned to @mclow

[jwakely]

Also:

#include <functional>

int f(short) noexcept { return 0; }
std::reference_wrapper<decltype(f)> r(f);
static_assert(std::is_same<decltype(r)::result_type, int>::value, "");
static_assert(std::is_same<decltype(r)::argument_type, short>::value, "");

auto* p = &f;
std::reference_wrapper<decltype(&f)> r2(p);
static_assert(std::is_same<decltype(r2)::result_type, int>::value, "");
static_assert(std::is_same<decltype(r2)::argument_type, short>::value, "");

struct X { long f() const & noexcept { return 0L; } };
auto m = &X::f;
std::reference_wrapper<decltype(m)> r3(m);
static_assert(std::is_same<decltype(r3)::result_type, long>::value, "");
static_assert(std::is_same<decltype(r3)::argument_type, const X*>::value, "");

auto b = std::bind(f, 1);
static_assert(std::is_same<decltype(b)::result_type, int>::value, "");

auto b2 = std::bind(&X::f, X{});
static_assert(std::is_same<decltype(b2)::result_type, long>::value, "");

This requires support for functions with ref-qualifiers and (in C++17 and above) noexcept.

[frederick-vs-ja]

The following is also incorrectly rejected. Godbolt link.

#include <functional>

struct Bar { using result_type = int&; };
struct Baz { using result_type = int() const&; };

using R1 = std::reference_wrapper<Bar>::result_type;
using R2 = std::reference_wrapper<Baz>::result_type;

[frederick-vs-ja]

Looks like that this hasn't been touched for years. I think I'll fix this once #169870 is merged.