Class library assembly working in Unity but not in VS2017

We're exploring using Zip files in our project (whether or not this is the right thing to do is another discussion!) The class of interest is System.IO.Compression.ZipArchiveEntry

Following the instructions at https://docs.unity3d.com/Manual/dotnetProfileAssemblies.html and
https://docs.microsoft.com/en-us/visualstudio/cross-platform/unity-scripting-upgrade?view=vs-2017 we've created a response file whose contents are:

-r:System.IO.Compression.dll
-r:System.IO.Compression.FileSystem.dll

Unity itself is then perfectly happy with references to ZipArchiveEntry, but VS2017 complains:
error CS1069: The type name 'ZipArchiveEntry' could not be found in the namespace 'System.IO.Compression'. This type has been forwarded to assembly 'System.IO.Compression, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089' Consider adding a reference to that assembly.

The scripting runtime is set to .NET 4.x Equivalent, backend is IL2CPP and API compatibility .NET 4.x

 

Surprised that nobody else has run into this. I tried upgrading to the latest 2019.11 alpha to no avail. Can't work out if it's a problem with Visual Studio itself, or the solution file created by Unity.