Fix NoWarn ignored when MSBuildWarningsAsMessages is set

[Copilot]

Fixes #12808

Context

When a project sets both MSBuildWarningsAsMessages (e.g., NU1701) and NoWarn (e.g., NETSDK1233), NoWarn is ignored because the targets only set MSBuildWarningsAsMessages from $(NoWarn) when MSBuildWarningsAsMessages is empty.

Changes Made

• Modified Microsoft.Common.CurrentVersion.targets to always append $(NoWarn) to $(MSBuildWarningsAsMessages):

<MSBuildWarningsAsMessages>$(MSBuildWarningsAsMessages);$(NoWarn)</MSBuildWarningsAsMessages>

• Added three end-to-end tests that import Common targets to verify the behavior:
  • NoWarnHonoredWhenMSBuildWarningsAsMessagesIsSetViaTargets: Verifies NoWarn is honored when MSBuildWarningsAsMessages is already set
  • NoWarnWorksWhenMSBuildWarningsAsMessagesIsNotSetViaTargets: Ensures original behavior is preserved when MSBuildWarningsAsMessages is not set
  • DuplicateDiagnosticCodesInNoWarnAndMSBuildWarningsAsMessagesAreHandled: Verifies that duplicate diagnostic codes (when the same code appears in both NoWarn and MSBuildWarningsAsMessages) are handled correctly

Testing

• All 28 existing WarningsAsMessagesAndErrors tests pass
• New tests verify: NoWarn honored when MSBuildWarningsAsMessages is set, original behavior preserved when it's not, and duplicate diagnostic codes are handled correctly

Notes

The leading semicolon when MSBuildWarningsAsMessages is initially empty (producing ;NAT013) is correctly handled by SemiColonTokenizer, which discards empty segments. This pattern is consistent with other property appending in the targets (e.g., AssemblySearchPaths).

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[Copilot]

Pull request overview

This PR fixes issue #12808 where the NoWarn property was being ignored when MSBuildWarningsAsMessages was already set. The fix modifies Microsoft.Common.CurrentVersion.targets to unconditionally append $(NoWarn) to $(MSBuildWarningsAsMessages) instead of only setting it when MSBuildWarningsAsMessages is empty.

Key Changes:

• Changed the property assignment from conditional to unconditional append in Microsoft.Common.CurrentVersion.targets
• Added two end-to-end tests that import Common targets to verify both the bug fix and backward compatibility

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/Tasks/Microsoft.Common.CurrentVersion.targets	Modified line 669 to always append $(NoWarn) to $(MSBuildWarningsAsMessages) using semicolon concatenation, removing the conditional check that prevented this when MSBuildWarningsAsMessages was already set
src/Build.UnitTests/WarningsAsMessagesAndErrors_Tests.cs	Added two test methods: one verifying that NoWarn is honored when MSBuildWarningsAsMessages is already set (the bug fix), and another ensuring the original behavior is preserved when MSBuildWarningsAsMessages is not set

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[baronfel]

@copilot ensure that there are tests that cover duplicate diagnostic code handling - now that NoWarn is being mirrored into MSbuildWarningsAsMessages it's possible that users will have a code set in both properties, so we need to be able to handle that.

[Copilot]

@copilot ensure that there are tests that cover duplicate diagnostic code handling - now that NoWarn is being mirrored into MSbuildWarningsAsMessages it's possible that users will have a code set ...

Added DuplicateDiagnosticCodesInNoWarnAndMSBuildWarningsAsMessagesAreHandled test in 27f3f04. This test verifies that when NAT012 appears in both NoWarn and MSBuildWarningsAsMessages, all warnings (including the duplicate) are correctly treated as messages.

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