Provide type trait or concept to verify if a projection type derives from another

[sylveon]

It might be useful in cases where a template uses a C++/WinRT projection type, to constrain that template to only types deriving from another projection type.

This can currently be achieved multiple ways:

template<typename T>
	requires std::derived_from<T, winrt::impl::base_one<T, winrt::Windows::UI::Xaml::Controls::Page>>
class XamlPageHost final : public BaseXamlPageHost {

template<typename T>
	requires std::convertible_to<T, winrt::Windows::UI::Xaml::Controls::Page>
class XamlPageHost final : public BaseXamlPageHost {

The problem with solution 1 is that it taps into implementation details of C++/WinRT, which is undesirable for a consumer of the library, but it is a reliable way currently to detect if T derives from Page.
Solution 2 has the issue that if some other non-projection type offers a conversion operator to the desired projection type, the template will match.

To resolve this situation, there should be a winrt::is_derived_from<T, U> type trait (and winrt::is_derived_from_v shorthand) which would have a static constexpr member value that is true, if the projection type T derives from type U.

[kennykerr]

We could potentially offer a type trait that will tell you whether T is a WinRT type. You could then combine that with std::is_convertible to answer such a question. Do you have a production scenario where this is needed?

[sylveon]

I have a template which can host any page which is derived from TranslucentTB.Xaml.FramelessPage (which implements some common functionality, like allowing XAML to alter the window's title just by setting a property) in an XAML island.

I simply want to add some compile-time validation in here, because no member function named Title (nested in between a bunch of other template errors) isn't a very helpful error compared to type does not derive from TranslucentTB.Xaml.FramelessPage

[kennykerr]

Feel free to reopen if there's interest in exploring this further.

[sylveon]

If I where to implement this myself, what would be the better way to detect if a type is a projection?

[kennykerr]

Paging @DefaultRyan as your scenario seems to involve Xaml composition and that's a bit of a mix of projected and implementation types. Perhaps he has a suggestion beyond std::is_convertible.