E.g. consider the following:

namespace Component
{
    struct Test
    {
        IReference<Int32> member;
    };
}

Then the generated file winrt/impl/Component.2.h will include an operator== definition that looks something like:

    inline bool operator==(Test const& left, Test const& right) noexcept
    {
        return left.member == right.member;
    }

It seems that the intended functionality is that the above uses the operator== defined for IReference<T>:

    template <typename T>
    bool operator==(IReference<T> const& left, IReference<T> const& right)
    {
        if (get_abi(left) == get_abi(right))
        {
            return true;
        }

        if (!left || !right)
        {
            return false;
        }

        return left.Value() == right.Value();
    }

This works fine, however the issue lies in the fact that the above operator== for IReference<T> is defined in winrt/Windows.Foundation.h. This file is included by neither winrt/Component.h nor winrt/impl/Component.2.h. Both - curiously enough - include winrt/impl/Windows.Foundation.2.h, however the desired function definition is not present in this file.

The end result is that winrt/Windows.Foundation.h must be explicitly included before winrt/Component.h for it to work properly.