Inconsistency between LSL and non-LSL restore when a project reference is framework incompatible

[nkolev92]

The setup is the following:
A net45 Project.json project references a net46 Legacy Package Ref project.
Attached repro solution.
Included in the root directory of the solution is project.lock file

Solution_a381.zip

Lock file latest dev lsl mode.

{
  "version": 3,
  "targets": {
    ".NETFramework,Version=v4.5": {
      "NuGet.Versioning/1.0.7": {
        "type": "package",
        "compile": {
          "lib/portable-net40+win/NuGet.Versioning.dll": {}
        },
        "runtime": {
          "lib/portable-net40+win/NuGet.Versioning.dll": {}
        }
      },
      "PackageReferenceClassLibrary_6ee6/1.0.0": {
        "type": "project",
        "framework": "Unsupported,Version=v0.0",
        "dependencies": {
          "NuGet.Versioning": "1.0.7"
        }
      }
    },
    ".NETFramework,Version=v4.5/win": {
      "NuGet.Versioning/1.0.7": {
        "type": "package",
        "compile": {
          "lib/portable-net40+win/NuGet.Versioning.dll": {}
        },
        "runtime": {
          "lib/portable-net40+win/NuGet.Versioning.dll": {}
        }
      },
      "PackageReferenceClassLibrary_6ee6/1.0.0": {
        "type": "project",
        "framework": "Unsupported,Version=v0.0",
        "dependencies": {
          "NuGet.Versioning": "1.0.7"
        }
      }
    },
    ".NETFramework,Version=v4.5/win-anycpu": {
      "NuGet.Versioning/1.0.7": {
        "type": "package",
        "compile": {
          "lib/portable-net40+win/NuGet.Versioning.dll": {}
        },
        "runtime": {
          "lib/portable-net40+win/NuGet.Versioning.dll": {}
        }
      },
      "PackageReferenceClassLibrary_6ee6/1.0.0": {
        "type": "project",
        "framework": "Unsupported,Version=v0.0",
        "dependencies": {
          "NuGet.Versioning": "1.0.7"
        }
      }
    },
    ".NETFramework,Version=v4.5/win-x86": {
      "NuGet.Versioning/1.0.7": {
        "type": "package",
        "compile": {
          "lib/portable-net40+win/NuGet.Versioning.dll": {}
        },
        "runtime": {
          "lib/portable-net40+win/NuGet.Versioning.dll": {}
        }
      },
      "PackageReferenceClassLibrary_6ee6/1.0.0": {
        "type": "project",
        "framework": "Unsupported,Version=v0.0",
        "dependencies": {
          "NuGet.Versioning": "1.0.7"
        }
      }
    }
  },
  "libraries": {
    "NuGet.Versioning/1.0.7": {
      "sha512": "NTiuXbF1wNSSSahdmLGFwrpVbGgUq1tNoLVgWsI7iWNF4Hv98JTi/kTfZbP0vMKNfv6GtuV77jnfb3hcEymcTQ==",
      "type": "package",
      "path": "nuget.versioning/1.0.7",
      "files": [
        "lib/portable-net40+win/NuGet.Versioning.XML",
        "lib/portable-net40+win/NuGet.Versioning.dll",
        "nuget.versioning.1.0.7.nupkg.sha512",
        "nuget.versioning.nuspec"
      ]
    },
    "PackageReferenceClassLibrary_6ee6/1.0.0": {
      "type": "project",
      "path": "../PackageReferenceClassLibrary_6ee6/PackageReferenceClassLibrary_6ee6.csproj",
      "msbuildProject": "../PackageReferenceClassLibrary_6ee6/PackageReferenceClassLibrary_6ee6.csproj"
    }
  },
  "projectFileDependencyGroups": {
    "": [],
    ".NETFramework,Version=v4.5": []
  },
  "packageFolders": {
    "C:\\Users\\nikolev.REDMOND\\.nuget\\packages\\": {}
  },
  "project": {
    "restore": {
      "projectUniqueName": "C:\\Users\\nikolev.REDMOND\\Desktop\\Solution_a381\\BuildIntegratedClassLibrary_a16c\\BuildIntegratedClassLibrary_a16c.csproj",
      "projectName": "BuildIntegratedClassLibrary_a16c",
      "projectPath": "C:\\Users\\nikolev.REDMOND\\Desktop\\Solution_a381\\BuildIntegratedClassLibrary_a16c\\BuildIntegratedClassLibrary_a16c.csproj",
      "projectJsonPath": "C:\\Users\\nikolev.REDMOND\\Desktop\\Solution_a381\\BuildIntegratedClassLibrary_a16c\\project.json",
      "packagesPath": "C:\\Users\\nikolev.REDMOND\\.nuget\\packages\\",
      "outputPath": "C:\\Users\\nikolev.REDMOND\\Desktop\\Solution_a381\\BuildIntegratedClassLibrary_a16c\\obj\\",
      "projectStyle": "ProjectJson",
      "configFilePaths": [
        "C:\\Users\\nikolev.REDMOND\\AppData\\Roaming\\NuGet\\NuGet.Config",
        "C:\\Program Files (x86)\\NuGet\\Config\\Microsoft.VisualStudio.Offline.config"
      ],
      "sources": {
        "C:\\Program Files (x86)\\Microsoft SDKs\\NuGetPackages\\": {},
        "C:\\Users\\nikolev.REDMOND\\.dotnet\\NuGetFallbackFolder": {},
        "D:\\BLAAA": {},
        "https://api.nuget.org/v3/index.json": {},
        "https://dotnet.myget.org/F/dotnet-core/api/v3/index.json": {}
      },
      "frameworks": {
        "net45": {
          "projectReferences": {
            "C:\\Users\\nikolev.REDMOND\\Desktop\\Solution_a381\\PackageReferenceClassLibrary_6ee6\\PackageReferenceClassLibrary_6ee6.csproj": {
              "projectPath": "C:\\Users\\nikolev.REDMOND\\Desktop\\Solution_a381\\PackageReferenceClassLibrary_6ee6\\PackageReferenceClassLibrary_6ee6.csproj"
            }
          }
        }
      }
    },
    "frameworks": {
      "net45": {}
    },
    "runtimes": {
      "win": {
        "#import": []
      },
      "win-anycpu": {
        "#import": []
      },
      "win-x86": {
        "#import": []
      }
    }
  },
  "logs": [
    {
      "code": "NU1201",
      "level": "Error",
      "message": "Project PackageReferenceClassLibrary_6ee6 is not compatible with net45 (.NETFramework,Version=v4.5). Project PackageReferenceClassLibrary_6ee6 supports: net46 (.NETFramework,Version=v4.6)",
      "libraryId": "PackageReferenceClassLibrary_6ee6",
      "targetGraphs": [
        ".NETFramework,Version=v4.5"
      ]
    },
    {
      "code": "NU1201",
      "level": "Error",
      "message": "Project PackageReferenceClassLibrary_6ee6 is not compatible with net45 (.NETFramework,Version=v4.5) / win. Project PackageReferenceClassLibrary_6ee6 supports: net46 (.NETFramework,Version=v4.6)",
      "libraryId": "PackageReferenceClassLibrary_6ee6",
      "targetGraphs": [
        ".NETFramework,Version=v4.5/win"
      ]
    },
    {
      "code": "NU1201",
      "level": "Error",
      "message": "Project PackageReferenceClassLibrary_6ee6 is not compatible with net45 (.NETFramework,Version=v4.5) / win-anycpu. Project PackageReferenceClassLibrary_6ee6 supports: net46 (.NETFramework,Version=v4.6)",
      "libraryId": "PackageReferenceClassLibrary_6ee6",
      "targetGraphs": [
        ".NETFramework,Version=v4.5/win-anycpu"
      ]
    },
    {
      "code": "NU1201",
      "level": "Error",
      "message": "Project PackageReferenceClassLibrary_6ee6 is not compatible with net45 (.NETFramework,Version=v4.5) / win-x86. Project PackageReferenceClassLibrary_6ee6 supports: net46 (.NETFramework,Version=v4.6)",
      "libraryId": "PackageReferenceClassLibrary_6ee6",
      "targetGraphs": [
        ".NETFramework,Version=v4.5/win-x86"
      ]
    }
  ]
}

Latest dev non-lsl mode

{
  "version": 3,
  "targets": {
    ".NETFramework,Version=v4.5": {},
    ".NETFramework,Version=v4.5/win": {},
    ".NETFramework,Version=v4.5/win-anycpu": {},
    ".NETFramework,Version=v4.5/win-x86": {}
  },
  "libraries": {},
  "projectFileDependencyGroups": {
    "": [],
    ".NETFramework,Version=v4.5": []
  },
  "packageFolders": {
    "C:\\Users\\nikolev.REDMOND\\.nuget\\packages\\": {}
  },
  "project": {
    "restore": {
      "projectUniqueName": "C:\\Users\\nikolev.REDMOND\\Desktop\\Solution_a381\\BuildIntegratedClassLibrary_a16c\\BuildIntegratedClassLibrary_a16c.csproj",
      "projectName": "BuildIntegratedClassLibrary_a16c",
      "projectPath": "C:\\Users\\nikolev.REDMOND\\Desktop\\Solution_a381\\BuildIntegratedClassLibrary_a16c\\BuildIntegratedClassLibrary_a16c.csproj",
      "projectJsonPath": "C:\\Users\\nikolev.REDMOND\\Desktop\\Solution_a381\\BuildIntegratedClassLibrary_a16c\\project.json",
      "packagesPath": "C:\\Users\\nikolev.REDMOND\\.nuget\\packages\\",
      "outputPath": "C:\\Users\\nikolev.REDMOND\\Desktop\\Solution_a381\\BuildIntegratedClassLibrary_a16c\\obj\\",
      "projectStyle": "ProjectJson",
      "configFilePaths": [
        "C:\\Users\\nikolev.REDMOND\\AppData\\Roaming\\NuGet\\NuGet.Config",
        "C:\\Program Files (x86)\\NuGet\\Config\\Microsoft.VisualStudio.Offline.config"
      ],
      "sources": {
        "C:\\Program Files (x86)\\Microsoft SDKs\\NuGetPackages\\": {},
        "C:\\Users\\nikolev.REDMOND\\.dotnet\\NuGetFallbackFolder": {},
        "D:\\BLAAA": {},
        "https://api.nuget.org/v3/index.json": {},
        "https://dotnet.myget.org/F/dotnet-core/api/v3/index.json": {}
      }
    },
    "frameworks": {
      "net45": {}
    },
    "runtimes": {
      "win": {
        "#import": []
      },
      "win-anycpu": {
        "#import": []
      },
      "win-x86": {
        "#import": []
      }
    }
  }
}

The 15.3 behavior is like the non-LSL one for both cases, however someone seemed to suggest this might not be the right thing.

We need to clarify the correct behavior here in general and add tests to verify that all works we think it does/should

[nkolev92]

While looking into figuring out the issue here, I've noticed that dg specs are different in the LSL and non-LSL mode.
The non-LSL one the 15.3 behavior and imho it should stay that way.
The zip file contain both dgs.
Investigating why the dg specs are different.
nuget-dg.zip

[nkolev92]

The inconsistency is that DeferredProjectWorkspaceService returns project reference despite the fact it's not compatible.

https://github.com/NuGet/NuGet.Client/blob/b892fe82d3435ac5fcae0ec664efc4f40dd0bcb7/src/NuGet.Clients/NuGet.PackageManagement.VisualStudio/Services/DeferredProjectWorkspaceService.cs#L55-L61

In the regular restore case:

It goes here for the project reference:
https://github.com/NuGet/NuGet.Client/blob/dev/src/NuGet.Clients/NuGet.PackageManagement.VisualStudio/ProjectServices/VsCoreProjectSystemReferenceReader.cs#L70-L75

[zhili1208]

it's risky to change PC and PJ behavior and LSL did right thing to throw error, close it as won't fix