Avoid constraint recursion for expected<any, ...> and expected<void, any>

[achabense]

Fixes #4011.

[achabense]

additional test

#include<any>
#include<expected>
#include<functional>

using namespace std;

void test_A() {
    using T = expected<any, int>;
    T t1;
    T t2{ move(t1) };
    T t3{ t2 };

    using U = expected<void, any>;
    U u1;
    U u2{ move(u1) };
    U u3{ u2 };
}

using X = expected<any, int>;
X meow() {
    return {};
}

void test_B() {
    function<X()> f{ meow };
}

int main() {
    test_A();
    test_B();
}

[CaseyCarter]

Some comments that I'll test-and-push changes for. Thanks for jumping on this bug so fast!

[StephanTLavavej]

Thanks! @CaseyCarter I pushed a trivial stylistic change after you approved.

[StephanTLavavej]

I'm mirroring this to the MSVC-internal repo - please notify me if any further changes are pushed.

[achabense]

Sorry for my untimely reply; if it's late to make new pushes and these changes look ok, I will carry these changes to another pr.

1. I think it's beneficial not to relocate _Allow_unwrapping here and in expected<void, ...>, as it is used closely after definition so its function can be more obvious.
2. These can also be private:
   

   STL/stl/inc/expected

   Line 351 in 6c69a73

   struct _NODISCARD _GuardTy {

   
   

   STL/stl/inc/expected

   Line 363 in 6c69a73

   static constexpr void _Reinit_expected(_First& _New_val, _Second& _Old_val, _Args&&... _Vals) noexcept(

[achabense]

I find another weird case... t contains U instead of int after construction; I think this is a bit counterintuitive...
(gcc and clang also think t should contain U)

#include<any>
#include<expected>
using namespace std;

int main() {
    using T = expected<any, int>;
    using U = expected<int, int>;

    T t{ U{} };
    (void) any_cast<U>(t.value()); // not thrown, t.any contains U instead of int
}

[frederick-vs-ja]

I find another weird case... t contains U instead of int after construction; I think this is a bit counterintuitive... (gcc and clang also think t should contain U)

#include<any>
#include<expected>
using namespace std;

int main() {
    using T = expected<any, int>;
    using U = expected<int, int>;

    T t{ U{} };
    (void) any_cast<U>(t.value()); // not thrown, t.any contains U instead of int
}

This is probably mandatory as is_constructible_v<unexpected<any>, expected<int, int>> is true. An LWG issue might be welcome but I currently have no idea on the resolution...

[StephanTLavavej]

Thanks for fixing this constraint recursion! 🔁 🛠️ 🎉