sdk: Cannot target uap10.0 apps with new TargetFrameworks syntax

Forgive me if this is already formally bugged somewhere, but while I’ve seen lots of references to uap10 targeting issues, I didn’t come across an issues that specifically addressed <targetframeworks> support.

While in general the documentation for TargetFrameworks and the new .csproj format seems to be, shall we say, on the skimpier side; I’m unable to devise whether or not - in theory - targeting UAP10.0 should work in a multi-targeted csproj.

I know UWP/UAP support is a bit of a thorn in the side of the dotnet sdk and corefx teams, but it’s very unclear as to how exactly it fits in to the picture.

A .NET library created with <TargetFrameworks>netstandard1.3</TargetFrameworks> can be consumed by a “legacy” UWP/UAP10 project, but there does not seem to be a way of creating a multi-targeted .NET library that includes a dependency on UWP.

Either UWP is a framework target, in which case <TargetFrameworks>uap10.0</TargetFrameworks> should work, or UWP is a .NET Standard target w/ access to Windows-specific and platform-specific libraries (which is what is most-heavily implied in the .NET Standard documentation). However, functionally that does not seem to be the case since a .NET Standard library (<TargetFrameworks>netstandard1.3</TargetFrameworks>) cannot install microsoft.netcore.universalwindowsplatform as a dependency.

We’re left in a limbo state where it seems projects targeting UAP/UWP must use the legacy csproj format, because they’re neither full-fledged framework targets nor do they have .NET Standard-compliant libraries.