OutputPath contains double path separator leading to NU5019 in nuget pack

[pharring]

Describe the bug

.NET 8 Preview 4 SDK
Requires: A .NET SDK project with multi-targeting (TargetFrameworks plural) and a custom .nuspec
The OutputPath property ends up with a double slash (path separator) because PlatformName is undefined.
Nuget pack doesn't like the double-slash, leading to a NU5019 (file not found) error.

To Reproduce

Here's a gist:
https://gist.github.com/pharring/aef107238843a1c6c7ea9f0be87795d6

Copy the three files to a local directory and build it with dotnet build
Output:

Build FAILED.

C:\Program Files\dotnet\sdk\8.0.100-preview.4.23260.5\Sdks\NuGet.Build.Tasks.Pack\buildCrossTargeting\NuGet.Build.Tasks.Pack.targets(221,5): error NU5019: File not found: 'b
in\\Debug\foo.txt'. [C:\throwaway\foo2\foo.csproj]

Exceptions (if any)

Note that this repros only with multitargeting. If you change TargetFrameworks to TargetFramework (singular), then it works.
The problem seems to be that PlatformName is not set in the multitargeting case (it should default to AnyCpu) so OutputPath gets initialized with a double path separator.

Further technical details

• Output of dotnet --info:

.NET SDK:
 Version:   8.0.100-preview.4.23260.5
 Commit:    2268e7b15c

Runtime Environment:
 OS Name:     Windows
 OS Version:  10.0.22621
 OS Platform: Windows
 RID:         win10-x64
 Base Path:   C:\Program Files\dotnet\sdk\8.0.100-preview.4.23260.5\

.NET workloads installed:
There are no installed workloads to display.

Host:
  Version:      8.0.0-preview.4.23259.5
  Architecture: x64
  Commit:       84a3d0e37e

.NET SDKs installed:
  7.0.400-preview.23274.1 [C:\Program Files\dotnet\sdk]
  8.0.100-preview.4.23260.5 [C:\Program Files\dotnet\sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.All 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.All]
  Microsoft.AspNetCore.App 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 3.1.32 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 6.0.16 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 7.0.5 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.AspNetCore.App 8.0.0-preview.4.23260.4 [C:\Program Files\dotnet\shared\Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 2.1.30 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 3.1.32 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 6.0.16 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 7.0.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.NETCore.App 8.0.0-preview.4.23259.5 [C:\Program Files\dotnet\shared\Microsoft.NETCore.App]
  Microsoft.WindowsDesktop.App 3.1.32 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 5.0.17 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 6.0.16 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 7.0.5 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]
  Microsoft.WindowsDesktop.App 8.0.0-preview.4.23260.1 [C:\Program Files\dotnet\shared\Microsoft.WindowsDesktop.App]

Other architectures found:
  x86   [C:\Program Files (x86)\dotnet]
    registered at [HKLM\SOFTWARE\dotnet\Setup\InstalledVersions\x86\InstallLocation]

Environment variables:
  Not set

global.json file:
  Not found

• The IDE (VS / VS Code/ VS4Mac) you're running on, and its version: VS 2022 17.7 Preview 2

[pharring]

I should note that this is a regression from the .NET 7 SDK.
The repro was distilled from build failure I encountered in the TraceEvent library (part of PerfView): https://github.com/microsoft/perfview/blob/main/src/TraceEvent/TraceEvent.csproj

Thanks for creating this issue! We believe this issue is related to NuGet tooling, which is maintained by the NuGet team. Thus, we closed this one and encourage you to raise this issue in the NuGet repository instead. Don’t forget to check out NuGet’s contributing guide before submitting an issue!

If you believe this issue was closed out of error, please comment to let us know.

Happy Coding!

[pharring]

NuGet team says this is a .NET SDK regression: NuGet/Home#12628 (comment)

[baronfel]

@dsplaisted NuGet thinks this is related to the output path changes, can you check?

[dsplaisted]

Thanks folks for the report, this was an SDK regression. I've sent #33003 to fix it.