Improve diagnostics when implements<> is used incorectly

[oldnewthing]

A common error is forgetting to provide the name of the derived class as the first implements<> template parameter. This results in a strange 'implements_type' is not a member of any direct or indirect base class of 'MyClass' error. Detect this case with a static_assert with a more help error message.

Note that we have to put the static_assert inside the implementation of GetRuntimeClassName rather than putting it in the class definition of implements. That's because implements is sometimes used with an incomplete type, and we cannot use is_base_of with incomplete types. Wait until GetRuntimeClassName is generated, at which point the class is complete.

[kennykerr]

Same here: I kicked off a build - there were vpack issues, but otherwise the build was successful.

What's the error look like?

[oldnewthing]

struct Foo : winrt::implements<winrt::Windows::Foundation::IStringable, winrt::Windows::Foundation::IClosable>
{  //<< Line 25
    void Close() { }
    winrt::hstring ToString() { return L""; }
};

void test()
{
    auto foo = winrt::make<Foo>();
}

Before

Severity	Code	Description	Project	File	Line
Error	C2794	'implements_type': is not a member of any direct or indirect base class of 'winrt::Windows::Foundation::IStringable'	test	base.h	6613

base.h(6613,69): error C2794: 'implements_type': is not a member of any direct or indirect base class of 'winrt::Windows::Foundation::IStringable'
base.h(6633): message : see reference to alias template instantiation 'winrt::impl::implemented_interfaces<winrt::Windows::Foundation::IStringable>' being compiled
base.h(7707): message : see reference to class template instantiation 'winrt::impl::implements_default_interface<D,void>' being compiled
1>        with
1>        [
1>            D=winrt::Windows::Foundation::IStringable
1>        ]
base.h(7707): message : while compiling class template member function 'winrt::hstring winrt::implements<winrt::Windows::Foundation::IStringable,winrt::Windows::Foundation::IClosable>::GetRuntimeClassName(void) const'
test.cpp(25): message : see reference to class template instantiation 'winrt::implements<winrt::Windows::Foundation::IStringable,winrt::Windows::Foundation::IClosable>' being compiled

base.h(6613) is

    template <typename T>
    using implemented_interfaces = filter<is_interface, typename T::implements_type>;

which doesn't really give the developer much to work from.

After

Severity	Code	Description	Project	File	Line
Error	C2338	Class must derive from implements<> or ClassT<> where the first template parameter is the derived class name, e.g. struct D : implements<D, ...>	test	base.h	7708

base.h(7708,32): error C2338: Class must derive from implements<> or ClassT<> where the first template parameter is the derived class name, e.g. struct D : implements<D, ...>
base.h(7707): message : while compiling class template member function 'winrt::hstring winrt::implements<winrt::Windows::Foundation::IStringable,winrt::Windows::Foundation::IClosable>::GetRuntimeClassName(void) const'
test.cpp(25): message : see reference to class template instantiation 'winrt::implements<winrt::Windows::Foundation::IStringable,winrt::Windows::Foundation::IClosable>' being compiled

Line 7708 is

static_assert(std::is_base_of_v<implements_type, D>, "Class must derive from implements<> or ClassT<> where the first template parameter is the derived class name, e.g. struct D : implements < D, ...>");

This is then followed by the "Before" error.

Note

We still have an inscrutable error if you write

struct Foo : winrt::implements<winrt::Windows::Foundation::IClosable>
{ ... };

In that case you get

Severity	Code	Description	Project	File	Line
Error	C2039	'first_interface': is not a member of 'winrt::impl::interface_list<>'	test	base.h	6633

at

        using type = typename default_interface<typename implemented_interfaces<T>::first_interface>::type;

Still working on that one.