Test It_throws_a_warning_when_NuGetPackageRoot_is_empty fails on release/10.0.3xx due to NuGet 7.6 behavior change

[nagilson]

Summary

The test It_throws_a_warning_when_NuGetPackageRoot_is_empty in GivenThatWeWantToUseFrameworkRoslyn.cs is consistently failing on all PRs targeting release/10.0.3xx for 11+ days. The test passes on main.

Details

The test expects NETSDK1221 to be emitted when NuGetPackageRoot is empty during combined /t:Restore;Build with Full Framework MSBuild. However, on the release/10.0.3xx SDK (which bundles NuGet 7.6.0), NuGetPackageRoot is no longer empty at evaluation time — it appears to be pre-populated from the NUGET_PACKAGES environment variable that the test sets.

On main (SDK 11.x, NuGet 7.9.0): NuGetPackageRoot is empty at evaluation → warning fires → test passes ✅
On release/10.0.3xx (SDK 10.0.3xx, NuGet 7.6.0): NuGetPackageRoot is NOT empty at evaluation → no warning → test fails ❌

Both use the same MSBuild 18.7.1 (VS 18 Insiders) on the Helix test machine. The SDK targets code is identical between branches.

Evidence

• Build 1467087 (PR [release/10.0.3xx] Source code updates from dotnet/dotnet #54506, post-pin, Jun 16): FullFramework fails with this test
• Build 1457691 (PR [release/10.0.3xx] Source code updates from dotnet/dotnet #54506, pre-pin, Jun 10): Same failure
• Build 1465783 (PR [release/10.0.3xx] Enable hot reloading for Blazor inside an iframe #53297, unrelated backport): Same failure
• Main build 1468021 (Jun 17): FullFramework passes, this test passes
• Helix job 20d94c9e-9551-4fc7-9943-e24477f646cf console log shows build succeeds with 0 warnings

Root Cause Hypothesis

NuGet 7.6.0 (bundled in SDK 10.0.3xx) appears to initialize NuGetPackageRoot from the NUGET_PACKAGES environment variable during evaluation, which makes the property non-empty even for fresh projects with no .nuget.g.props. NuGet 7.9.0 (bundled on main) does not do this, preserving the old behavior where NuGetPackageRoot remains empty until .nuget.g.props is imported.

This means the underlying bug (issue #43016) may be fixed in NuGet 7.6.0, making the NETSDK1221 warning unnecessary in this scenario.

Possible Fixes

1. Update the test to not set NUGET_PACKAGES env var (so NuGetPackageRoot is truly empty regardless of NuGet version)
2. Condition the test on the NuGet version behavior
3. Remove the test if the underlying issue is genuinely fixed by NuGet 7.6

Context

• Original implementation: PR Warn when NuGetPackageRoot is empty but needed for the toolset compiler #43119 by @jjonescz (commit 05a82501a985, Sep 2024)
• Linked issue: Microsoft.Net.Sdk.Compilers.Toolset download check fails because NuGetPackageRoot is unset in some scenarios #43016 (also filed by @jjonescz)
• Follow-up fix: commit e5f35a28bbfc adding -p:DOTNET_HOST_PATH= args (Apr 2025, also @jjonescz)
• This is blocking ALL codeflow and backport PRs targeting release/10.0.3xx

/cc @jjonescz

[jjonescz]

See bb5c91f and 3caf7d4. Feel free to cherry-pick where necessary.

[jjonescz]

For 10.0.3xx, I opened #54829.

[jjonescz]

For 10.0.4xx, this should be merged (it carries the fix), but someone other than me needs to approve it: #54776

[nagilson]

Thanks for jumping in on this.