Don't offer to simplify lambdas if it would change semantics

src/Analyzers/CSharp/Analyzers/RemoveUnnecessaryLambdaExpression/CSharpRemoveUnnecessaryLambdaExpressionDiagnosticAnalyzer.cs

@@ -176,6 +176,26 @@ private void AnalyzeSyntax(SyntaxNodeAnalysisContext context, INamedTypeSymbol?
             if (invokedMethod.GetAttributes().Any(a => Equals(a.AttributeClass, conditionalAttributeType)))
                 return;
 
+            // In the case where we have `() => expr.m()`, check if `expr` is overwritten anywhere. If so then we do not
+            // want to remove the lambda, as that will bind eagerly to the original `expr` and will not see the write
+            // that later happens
+            if (invokedExpression is MemberAccessExpressionSyntax { Expression: var accessedExpression })
+            {
+                // Limit the search space to the outermost code block that could contain references to this expr (or
+                // fall back to compilation unit for top level statements).
+                var outermostBody = invokedExpression.AncestorsAndSelf().Last(
+                    n => n is BlockSyntax or ArrowExpressionClauseSyntax or AnonymousFunctionExpressionSyntax or CompilationUnitSyntax);
+                foreach (var candidate in outermostBody.DescendantNodes().OfType<ExpressionSyntax>())
+                {
+                    if (candidate != accessedExpression &&
+                        SemanticEquivalence.AreEquivalent(semanticModel, candidate, accessedExpression) &&
+                        candidate.IsWrittenTo(semanticModel, cancellationToken))
+                    {
+                        return;
+                    }
+                }
+            }
+
             // Semantically, this looks good to go.  Now, do an actual speculative replacement to ensure that the
             // non-invoked method reference refers to the same method symbol, and that it converts to the same type that
             // the lambda was.

src/Analyzers/CSharp/Tests/RemoveUnnecessaryLambdaExpression/RemoveUnnecessaryLambdaExpressionTests.cs

@@ -21,18 +21,29 @@ namespace Microsoft.CodeAnalysis.Editor.CSharp.UnitTests.RemoveUnnecessaryLambda
     [Trait(Traits.Feature, Traits.Features.CodeActionsRemoveUnnecessaryLambdaExpression)]
     public class RemoveUnnecessaryLambdaExpressionTests
     {
-        private static async Task TestInRegularAndScriptAsync(string testCode, string fixedCode, LanguageVersion version = LanguageVersion.Preview)
+        private static async Task TestInRegularAndScriptAsync(
+            string testCode,
+            string fixedCode,
+            LanguageVersion version = LanguageVersion.Preview,
+            OutputKind? outputKind = null)
         {
             await new VerifyCS.Test
             {
                 TestCode = testCode,
                 FixedCode = fixedCode,
                 LanguageVersion = version,
+                TestState =
+                {
+                    OutputKind = outputKind,
+                }
             }.RunAsync();
         }
 
-        private static Task TestMissingInRegularAndScriptAsync(string testCode, LanguageVersion version = LanguageVersion.Preview)
-            => TestInRegularAndScriptAsync(testCode, testCode, version);
+        private static Task TestMissingInRegularAndScriptAsync(
+            string testCode,
+            LanguageVersion version = LanguageVersion.Preview,
+            OutputKind? outputKind = null)
+            => TestInRegularAndScriptAsync(testCode, testCode, version, outputKind);
 
         [Fact]
         public async Task TestMissingInCSharp10()
@@ -1722,5 +1733,242 @@ private static void M2(Action<string> a) { }
                 }
                 """);
         }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/69094")]
+        public async Task TestNotWithAssignmentOfInvokedExpression1()
+        {
+            await TestMissingInRegularAndScriptAsync("""
+                using System;
+                using System.Threading.Tasks;
+
+                TaskCompletionSource<bool> valueSet = new();
+                Helper helper = new(v => valueSet.SetResult(v));
+                helper.Set(true);
+                valueSet = new();
+                helper.Set(false);
+
+                class Helper
+                {
+                   private readonly Action<bool> action;
+                   internal Helper(Action<bool> action)
+                   {
+                     this.action = action;
+                   }
+
+                   internal void Set(bool value) => action(value);
+                }
+
+                """, outputKind: OutputKind.ConsoleApplication);
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/69094")]
+        public async Task TestWithoutAssignmentOfInvokedExpression1()
+        {
+            await TestInRegularAndScriptAsync("""
+                using System;
+                using System.Threading.Tasks;
+
+                TaskCompletionSource<bool> valueSet = new();
+                Helper helper = new([|v => |]valueSet.SetResult(v));
+                helper.Set(true);
+                helper.Set(false);
+
+                class Helper
+                {
+                   private readonly Action<bool> action;
+                   internal Helper(Action<bool> action)
+                   {
+                     this.action = action;
+                   }
+
+                   internal void Set(bool value) => action(value);
+                }
+
+                """, """
+                using System;
+                using System.Threading.Tasks;
+
+                TaskCompletionSource<bool> valueSet = new();
+                Helper helper = new(valueSet.SetResult);
+                helper.Set(true);
+                helper.Set(false);
+
+                class Helper
+                {
+                   private readonly Action<bool> action;
+                   internal Helper(Action<bool> action)
+                   {
+                     this.action = action;
+                   }
+
+                   internal void Set(bool value) => action(value);
+                }
+
+                """,
+                outputKind: OutputKind.ConsoleApplication);
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/69094")]
+        public async Task TestNotWithAssignmentOfInvokedExpression2()
+        {
+            await TestMissingInRegularAndScriptAsync("""
+                using System;
+                using System.Threading.Tasks;
+
+                class C
+                {
+                    void M()
+                    {
+                        TaskCompletionSource<bool> valueSet = new();
+                        Helper helper = new(v => valueSet.SetResult(v));
+                        helper.Set(true);
+                        valueSet = new();
+                        helper.Set(false);
+                    }
+                }
+
+                class Helper
+                {
+                   private readonly Action<bool> action;
+                   internal Helper(Action<bool> action)
+                   {
+                     this.action = action;
+                   }
+
+                   internal void Set(bool value) => action(value);
+                }
+
+                """);
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/69094")]
+        public async Task TestWithoutAssignmentOfInvokedExpression2()
+        {
+            await TestInRegularAndScriptAsync("""
+                using System;
+                using System.Threading.Tasks;
+
+                class C
+                {
+                    void M()
+                    {
+                        TaskCompletionSource<bool> valueSet = new();
+                        Helper helper = new([|v => |]valueSet.SetResult(v));
+                        helper.Set(true);
+                        helper.Set(false);
+                    }
+                }
+
+                class Helper
+                {
+                   private readonly Action<bool> action;
+                   internal Helper(Action<bool> action)
+                   {
+                     this.action = action;
+                   }
+
+                   internal void Set(bool value) => action(value);
+                }
+
+                """, """
+                using System;
+                using System.Threading.Tasks;
+
+                class C
+                {
+                    void M()
+                    {
+                        TaskCompletionSource<bool> valueSet = new();
+                        Helper helper = new(valueSet.SetResult);
+                        helper.Set(true);
+                        helper.Set(false);
+                    }
+                }
+
+                class Helper
+                {
+                   private readonly Action<bool> action;
+                   internal Helper(Action<bool> action)
+                   {
+                     this.action = action;
+                   }
+
+                   internal void Set(bool value) => action(value);
+                }
+
+                """);
+        }
+
+        [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/69094")]
+        public async Task TestWithoutAssignmentOfInvokedExpression3()
+        {
+            await TestInRegularAndScriptAsync("""
+                using System;
+                using System.Threading.Tasks;
+
+                class C
+                {
+                    void M()
+                    {
+                        TaskCompletionSource<bool> valueSet = new();
+                        Helper helper = new([|v => |]valueSet.SetResult(v));
+                        helper.Set(true);
+                        helper.Set(false);
+
+                        var v = () =>
+                        {
+                            // this is a different local.  it should not impact the outer simplification
+                            TaskCompletionSource<bool> valueSet = new();
+                            valueSet = new();
+                        };
+                    }
+                }
+
+                class Helper
+                {
+                   private readonly Action<bool> action;
+                   internal Helper(Action<bool> action)
+                   {
+                     this.action = action;
+                   }
+
+                   internal void Set(bool value) => action(value);
+                }
+
+                """, """
+                using System;
+                using System.Threading.Tasks;
+
+                class C
+                {
+                    void M()
+                    {
+                        TaskCompletionSource<bool> valueSet = new();
+                        Helper helper = new(valueSet.SetResult);
+                        helper.Set(true);
+                        helper.Set(false);
+
+                        var v = () =>
+                        {
+                            // this is a different local.  it should not impact the outer simplification
+                            TaskCompletionSource<bool> valueSet = new();
+                            valueSet = new();
+                        };
+                    }
+                }
+
+                class Helper
+                {
+                   private readonly Action<bool> action;
+                   internal Helper(Action<bool> action)
+                   {
+                     this.action = action;
+                   }
+
+                   internal void Set(bool value) => action(value);
+                }
+
+                """);
+        }
     }
 }

src/Workspaces/SharedUtilitiesAndExtensions/Compiler/Core/CompilerExtensions.projitems

@@ -534,6 +534,7 @@
     <Compile Include="$(MSBuildThisFileDirectory)Utilities\ReferenceCountedDisposable.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Utilities\ReferenceCountedDisposableCache.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Utilities\RestrictedInternalsVisibleToAttribute.cs" />
+    <Compile Include="$(MSBuildThisFileDirectory)Extensions\SemanticEquivalence.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Utilities\SemaphoreSlimFactory.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Utilities\SerializableBytes.cs" />
     <Compile Include="$(MSBuildThisFileDirectory)Utilities\SoftCrashException.cs" />