`implements` class template triggers false positive code analysis warning

[alexlamtest]

Version

No response

Summary

PREFast scanning of a generated cppwinrt file for WASDK sample app /Samples/ResourceManagement/cpp-winui/winui_desktop_packaged_app_cpp/Generated Files/winrt/base.h on the https://github.com/microsoft/WindowsAppSDK-Samples repository hit warning: "'Temp_value_#22808' holds a value that must be examined."
The right fix for this issue appears to be centrally in cppwinrt generated code, instead of the individual sample apps that leverage cppwinrt.

Reproducible example

The following code in Base.h seems to be what the warning is referring to:
        impl::IWeakReferenceSource* make_weak_ref() noexcept
        {
            if constexpr (is_weak_ref_source::value)
            {
                uintptr_t count_or_pointer = m_references.load(std::memory_order_relaxed);

                if (is_weak_ref(count_or_pointer))
                {
                    return decode_weak_ref(count_or_pointer)->get_source();
                }

                com_ptr<weak_ref_t> weak_ref;
                *weak_ref.put() = new (std::nothrow) weak_ref_t(get_unknown(), static_cast<uint32_t>(count_or_pointer));

                if (!weak_ref)
                {
                    return nullptr;
                }

                uintptr_t const encoding = encode_weak_ref(weak_ref.get());

                while (true)
                {
                    if (m_references.compare_exchange_weak(count_or_pointer, encoding, std::memory_order_acq_rel, std::memory_order_relaxed))
                    {
                        impl::IWeakReferenceSource* result = weak_ref->get_source();
                        detach_abi(weak_ref);
                        return result;
                    }

                    if (is_weak_ref(count_or_pointer))
                    {
                        return decode_weak_ref(count_or_pointer)->get_source();
                    }

                    weak_ref->set_strong(static_cast<uint32_t>(count_or_pointer));
                }
            }
            else
            {
                static_assert(is_weak_ref_source::value, "Weak references are not supported because no_weak_ref was specified.");
                return nullptr;
            }
        }

Expected behavior

No response

Actual behavior

No response

Additional comments

No response

[kennykerr]

Thanks Alex, please provide a minimal reproducible example to verify the issue & fix as I'm not familiar with prefast.

[kennykerr]

Found an internal thread discussing this issue. Here's a minimal repro:

using namespace winrt;

struct s : implements<s, Windows::Foundation::IClosable>
{
    void Close() {}
};

int main()
{
}

An instantiation of the implements class template will trigger the following code analysis warning:

base.h(7830): warning C28193: 'Temp_value_#152' holds a value that must be examined.

Note that this is a false positive and should be fixed by MSVC but I suspect its affecting a lot of builds so I'll probably just provide a workaround.