Last night Microsoft released Windows 10 SDK Preview Build 17069 to developers. This SDK can be used with Windows 10 Insider Build 17074 or later. At the time the changelog has not been updated, but today Microsoft revealed what is new:
C++/WinRT Now Available: The C++/WinRT headers and cppwinrt compiler (cppwinrt.exe) are now included in the Windows SDK. The compiler comes in handy if you need to consume a third-party WinRT component or if you need to author your own WinRT components with C++/WinRT. The easiest way to get working with it after installing the Windows Insider Preview SDK is to start the Visual Studio Developer Command Prompt and run the compiler in that environment. Authoring support is currently experimental and subject to change. Stay tuned as we will publish more detailed instructions on how to use the compiler in the coming week.
The SDK is supported by Visual Studio 2017 and can work side-by-side with older SDKs.
The updated Preview SDK can be downloaded here and read more at Microsoft here about the new MIDL keywords and a roundup of new APIs added since 16299.