Fix ShouldTreatWarningAsError in OOP TaskHost checking wrong collection

documentation/wiki/ChangeWaves.md

@@ -31,6 +31,7 @@ Change wave checks around features will be removed in the release that accompani
 
 ### 18.6
 - [AbsolutePath.GetCanonicalForm optimization - avoid expensive Path.GetFullPath calls when paths don't need canonicalization](https://github.com/dotnet/msbuild/pull/13369)
+- [Fix ShouldTreatWarningAsError in OOP TaskHost checking wrong collection (WarningsAsMessages instead of WarningsAsErrors)](https://github.com/dotnet/msbuild/issues/11952)
 - [Fix ToolTask hang when tool spawns grandchild processes that inherit stdout/stderr pipe handles](https://github.com/dotnet/msbuild/issues/2981)
 
 ### 18.5

src/Build.UnitTests/WarningsAsMessagesAndErrors_Tests.cs

@@ -3,6 +3,7 @@
 
 using System;
 using System.Collections.Generic;
+using System.IO;
 using System.Linq;
 using Microsoft.Build.Framework;
 using Microsoft.Build.UnitTests;
@@ -18,6 +19,9 @@ public sealed class WarningsAsMessagesAndErrorsTests
     {
         private const string ExpectedEventMessage = "03767942CDB147B98D0ECDBDE1436DA3";
         private const string ExpectedEventCode = "0BF68998";
+        private static string TestAssemblyLocation { get; } = Path.Combine(
+            Path.GetDirectoryName(typeof(WarningsAsMessagesAndErrorsTests).Assembly.Location) ?? AppContext.BaseDirectory,
+            "Microsoft.Build.Engine.UnitTests.dll");
         private ITestOutputHelper _output;
 
         public WarningsAsMessagesAndErrorsTests(ITestOutputHelper output)
@@ -673,5 +677,86 @@ public void DuplicateDiagnosticCodesInNoWarnAndMSBuildWarningsAsMessagesAreHandl
                 logger.ErrorCount.ShouldBe(0);
             }
         }
+        /// <summary>
+        /// Verifies that when a task runs out-of-proc via TaskHostFactory and logs a warning
+        /// whose code is in MSBuildWarningsAsErrors, the warning is promoted to an error.
+        /// This was broken before ChangeWave 18.6 (the OOP code checked WarningsAsMessages
+        /// instead of WarningsAsErrors).
+        /// </summary>
+        [Fact]
+        public void TreatWarningsAsErrorsWhenSpecified_TaskHostFactory()
+        {
+            using TestEnvironment env = TestEnvironment.Create(_output);
+
+            TransientTestProjectWithFiles proj = env.CreateTestProjectWithFiles($@"
+                <Project>
+                    <UsingTask TaskName=""CustomLogAndReturnTask"" AssemblyFile=""{TestAssemblyLocation}"" TaskFactory=""TaskHostFactory"" />
+                    <PropertyGroup>
+                        <MSBuildWarningsAsErrors>{ExpectedEventCode}</MSBuildWarningsAsErrors>
+                    </PropertyGroup>
+                    <Target Name='Build'>
+                        <CustomLogAndReturnTask Return=""true"" WarningCode=""{ExpectedEventCode}""/>
+                    </Target>
+                </Project>");
+
+            MockLogger logger = proj.BuildProjectExpectFailure();
+
+            logger.ErrorCount.ShouldBe(1);
+            logger.WarningCount.ShouldBe(0);
+            logger.AssertLogContains(ExpectedEventCode);
+        }
+
+        /// <summary>
+        /// Verifies that when a task runs out-of-proc via TaskHostFactory and
+        /// MSBuildTreatWarningsAsErrors is true, all warnings are promoted to errors.
+        /// </summary>
+        [Fact]
+        public void TreatAllWarningsAsErrors_TaskHostFactory()
+        {
+            using TestEnvironment env = TestEnvironment.Create(_output);
+
+            TransientTestProjectWithFiles proj = env.CreateTestProjectWithFiles($@"
+                <Project>
+                    <UsingTask TaskName=""CustomLogAndReturnTask"" AssemblyFile=""{TestAssemblyLocation}"" TaskFactory=""TaskHostFactory"" />
+                    <PropertyGroup>
+                        <MSBuildTreatWarningsAsErrors>true</MSBuildTreatWarningsAsErrors>
+                    </PropertyGroup>
+                    <Target Name='Build'>
+                        <CustomLogAndReturnTask Return=""true"" WarningCode=""{ExpectedEventCode}""/>
+                    </Target>
+                </Project>");
+
+            MockLogger logger = proj.BuildProjectExpectFailure();
+
+            logger.ErrorCount.ShouldBe(1);
+            logger.WarningCount.ShouldBe(0);
+        }
+
+        /// <summary>
+        /// Verifies that WarningsAsMessages takes priority over WarningsAsErrors
+        /// when a task runs out-of-proc via TaskHostFactory.
+        /// </summary>
+        [Fact]
+        public void TreatWarningAsMessageOverridesTreatingItAsError_TaskHostFactory()
+        {
+            using TestEnvironment env = TestEnvironment.Create(_output);
+
+            TransientTestProjectWithFiles proj = env.CreateTestProjectWithFiles($@"
+                <Project>
+                    <UsingTask TaskName=""CustomLogAndReturnTask"" AssemblyFile=""{TestAssemblyLocation}"" TaskFactory=""TaskHostFactory"" />
+                    <PropertyGroup>
+                        <MSBuildWarningsAsErrors>{ExpectedEventCode}</MSBuildWarningsAsErrors>
+                        <MSBuildWarningsAsMessages>{ExpectedEventCode}</MSBuildWarningsAsMessages>
+                    </PropertyGroup>
+                    <Target Name='Build'>
+                        <CustomLogAndReturnTask Return=""true"" WarningCode=""{ExpectedEventCode}""/>
+                    </Target>
+                </Project>");
+
+            MockLogger logger = proj.BuildProjectExpectSuccess();
+
+            logger.ErrorCount.ShouldBe(0);
+            logger.WarningCount.ShouldBe(0);
+        }
     }
 }

src/MSBuild/OutOfProcTaskHostNode.cs

@@ -335,6 +335,13 @@ public bool ShouldTreatWarningAsError(string warningCode)
                 return false;
             }
 
+            if (ChangeWaves.AreFeaturesEnabled(ChangeWaves.Wave18_6))
+            {
+                // An empty set means all warnings are errors.
+                return (WarningsAsErrors.Count == 0 && WarningAsErrorNotOverriden(warningCode)) || WarningsAsErrors.Contains(warningCode);
+            }
+
+            // Pre-18.6 behavior preserved for backward compatibility: incorrectly checks WarningsAsMessages instead of WarningsAsErrors.
             return (WarningsAsErrors.Count == 0 && WarningAsErrorNotOverriden(warningCode)) || WarningsAsMessages.Contains(warningCode);
         }