MSBuild thinks a sln dependency is a reference

[lcsondes]

Steps to reproduce

I have a .NET 4.7 project which, as part of its build, runs a .NET Core CLI program to generate one of its source files using the BeforeBuild target and <Exec>. Because of this I want the generator to build before my project, so the .sln file has a dependency in the form of ProjectSection(ProjectDependencies) = postProject.

Expected behavior

The build works from within VS. I expect it to work from a command line MSBuild, because I in fact don't reference the .NET Core project, just call it as any other CLI tool. The fact that it's .NET Core is irrelevant, it could be C++.

Actual behavior

[GetReferenceNearestTargetFrameworkTask] C:\Program Files (x86)\Microsoft Visual Studio\2017\BuildTools\MSBuild\15.0\bin\amd64\Microsoft.Common.CurrentVersion.targets(1656, 5): Project 'C:\[snip...]\csharp_gen.csproj' targets 'netcoreapp2.1'. It cannot be referenced by a project that targets '.NETFramework,Version=v4.7'.

Environment data

msbuild /version output:

Microsoft (R) Build Engine version 15.7.179.6572 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

15.7.179.6572

OS info: Win7 x64, latest updates as of today

If applicable, version of the tool that invokes MSBuild (Visual Studio, dotnet CLI, etc):
msbuild.exe is being run standalone

[rainersigwald]

Duplicate of #1915. This should be fixed for 15.8 by #3315, and #2661 (comment) includes a workaround you can use today.