I'm using FSharp.Data.1.1.10 in a .NET 4.0 project, and when I add FSharp.Data with Nuget, it creates a packages\FSharp.Data.1.1.10\lib\net40 directory where it puts FSharp.Data.dll and FSharp.Data.DesignTime.dll. According to Telerik JustDecompile, FSharp.Data.dll is a .NET 4.0 dll, but FSharp.Data.DesignTime.dll is a .NET 4.5 dll even though it resides in the net40 subdirectory.
So, is FSharp.Data.DesignTime .NET 4.5 only, or is this a bug?