I'm on 17673, with VS 15.9 Preview 3 and RS5 SDK.
1>------ Build started: Project: XamlHostingSample, Configuration: Debug Win32 ------
1>Main.cpp
1>class template optional is only available with C++17 or later.
1>string_view is only available with C++17 or later.
1>c:\program files (x86)\windows kits\10\include\10.0.17763.0\um\windowsnumerics.impl.h(36): error C2429: language feature 'nested-namespace-definition' requires compiler flag '/std:c++17'
1>c:\program files (x86)\windows kits\10\include\10.0.17763.0\um\windowsnumerics.inl(137): error C2429: language feature 'nested-namespace-definition' requires compiler flag '/std:c++17'
1>c:\program files (x86)\windows kits\10\include\10.0.17763.0\cppwinrt\winrt\base.h(124): error C2429: language feature 'nested-namespace-definition' requires compiler flag '/std:c++17'
1>c:\program files (x86)\windows kits\10\include\10.0.17763.0\cppwinrt\winrt\base.h(142): error C2429: language feature 'nested-namespace-definition' requires compiler flag '/std:c++17'
1>c:\program files (x86)\windows kits\10\include\10.0.17763.0\cppwinrt\winrt\base.h(181): error C2429: language feature 'nested-namespace-definition' requires compiler flag '/std:c++17'
1>c:\program files (x86)\windows kits\10\include\10.0.17763.0\cppwinrt\winrt\base.h(397): error C2429: language feature 'nested-namespace-definition' requires compiler flag '/std:c++17'
1>c:\program files (x86)\windows kits\10\include\10.0.17763.0\cppwinrt\winrt\base.h(434): error C2429: language feature 'nested-namespace-definition' requires compiler flag '/std:c++17'
1>c:\program files (x86)\windows kits\10\include\10.0.17763.0\cppwinrt\winrt\base.h(481): error C2429: language feature 'nested-namespace-definition' requires compiler flag '/std:c++17'
1>------ Build started: Project: XamlHostingSample, Configuration: Debug Win32 ------
1>Main.cpp
1>c:\program files (x86)\windows kits\10\include\10.0.17763.0\cppwinrt\winrt\impl\windows.ui.xaml.media.animation.0.h(2016): warning C4002: too many arguments for function-like macro invocation 'GetCurrentTime'
1>Main.obj : error LNK2019: unresolved external symbol _WINRT_GetRestrictedErrorInfo@4 referenced in function "public: __thiscall winrt::hresult_error::hresult_error(struct winrt::hresult,struct winrt::hresult_error::from_abi_t)" (??0hresult_error@winrt@@QAE@Uhresult@1@Ufrom_abi_t@01@@Z)
1>Main.obj : error LNK2019: unresolved external symbol _WINRT_RoGetActivationFactory@12 referenced in function "__cdecl winrt::get_activation_factory<struct winrt::Windows::Foundation::IActivationFactory>(struct winrt::param::hstring const &)" (??$get_activation_factory@UIActivationFactory@Foundation@Windows@winrt@@@winrt@@YA@ABUhstring@param@0@@Z)
1>Main.obj : error LNK2019: unresolved external symbol _WINRT_RoInitialize@4 referenced in function "void __cdecl winrt::init_apartment(enum winrt::apartment_type)" (?init_apartment@winrt@@YAXW4apartment_type@1@@Z)
1>Main.obj : error LNK2019: unresolved external symbol _WINRT_RoOriginateLanguageException@12 referenced in function "private: void __thiscall winrt::hresult_error::originate(struct winrt::hresult,void *)" (?originate@hresult_error@winrt@@AAEXUhresult@2@PAX@Z)
1>Main.obj : error LNK2019: unresolved external symbol _WINRT_SetRestrictedErrorInfo@4 referenced in function "public: struct winrt::hresult __thiscall winrt::hresult_error::to_abi(void)const " (?to_abi@hresult_error@winrt@@QBE?AUhresult@2@XZ)
1>Main.obj : error LNK2019: unresolved external symbol _WINRT_WindowsCreateString@12 referenced in function "void * __cdecl winrt::impl::create_string(wchar_t const *,unsigned int)" (?create_string@impl@winrt@@YAPAXPB_WI@Z)
1>Main.obj : error LNK2019: unresolved external symbol _WINRT_WindowsCreateStringReference@16 referenced in function "public: __thiscall winrt::param::hstring::hstring(class std::basic_string_view<wchar_t,struct std::char_traits<wchar_t> > const &)" (??0hstring@param@winrt@@QAE@ABV?$basic_string_view@_WU?$char_traits@_W@std@@@std@@@Z)
1>Main.obj : error LNK2019: unresolved external symbol _WINRT_WindowsDeleteString@4 referenced in function "public: static void __cdecl winrt::impl::hstring_traits::close(void *)" (?close@hstring_traits@impl@winrt@@SAXPAX@Z)
1>Main.obj : error LNK2019: unresolved external symbol _WINRT_WindowsPreallocateStringBuffer@12 referenced in function "public: __thiscall winrt::impl::hstring_builder::hstring_builder(unsigned int)" (??0hstring_builder@impl@winrt@@QAE@I@Z)
1>Main.obj : error LNK2019: unresolved external symbol _WINRT_WindowsDeleteStringBuffer@4 referenced in function "public: __thiscall winrt::impl::hstring_builder::~hstring_builder(void)" (??1hstring_builder@impl@winrt@@QAE@XZ)
1>Main.obj : error LNK2019: unresolved external symbol _WINRT_WindowsPromoteStringBuffer@8 referenced in function "public: struct winrt::hstring __thiscall winrt::impl::hstring_builder::to_hstring(void)" (?to_hstring@hstring_builder@impl@winrt@@QAE?AUhstring@3@XZ)
1>Main.obj : error LNK2019: unresolved external symbol _WINRT_WindowsGetStringRawBuffer@8 referenced in function "public: __thiscall winrt::hstring::operator class std::basic_string_view<wchar_t,struct std::char_traits<wchar_t> >(void)const " (??Bhstring@winrt@@QBE?AV?$basic_string_view@_WU?$char_traits@_W@std@@@std@@XZ)
1>C:\code\Windows-appsample-Xaml-Hosting\Debug\XamlHostingSample.exe : fatal error LNK1120: 12 unresolved externals
1>Done building project "XamlHostingSample.vcxproj" -- FAILED.
========== Build: 0 succeeded, 1 failed, 0 up-to-date, 0 skipped ==========