Fix S2583 FP: Delegate unsubscription

analyzers/src/SonarAnalyzer.Common/SymbolicExecution/Roslyn/OperationProcessors/CompoundAssignment.cs

@@ -29,6 +29,7 @@ protected override ICompoundAssignmentOperationWrapper Convert(IOperation operat
 
     protected override ProgramState Process(SymbolicContext context, ICompoundAssignmentOperationWrapper assignment) =>
         ProcessNumericalCompoundAssignment(context.State, assignment)
+        ?? ProcessDelegateCompoundAssignment(context.State, assignment)
         ?? ProcessCompoundAssignment(context.State, assignment)
         ?? context.State;
 
@@ -51,6 +52,28 @@ private static ProgramState ProcessNumericalCompoundAssignment(ProgramState stat
         }
     }
 
+    private static ProgramState ProcessDelegateCompoundAssignment(ProgramState state, ICompoundAssignmentOperationWrapper assignment)
+    {
+        // When the -= operator is used on a delegate instance (for unsubscribing),
+        // it can leave the invocation list associated with the delegate empty. In that case the delegate instance will become null.
+        // https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/operators/subtraction-operator#delegate-removal
+        // For this reason, we remove the NotNull constraint from the delegate instance.
+        if (assignment.Target.Type.TypeKind == TypeKind.Delegate
+            && assignment.OperatorKind == BinaryOperatorKind.Subtract
+            && state.HasConstraint(assignment.Target, ObjectConstraint.NotNull))
+        {
+            var value = (state[assignment.Target] ?? SymbolicValue.Empty).WithoutConstraint(ObjectConstraint.NotNull);
+            return state
+                .SetOperationValue(assignment, value)
+                .SetOperationValue(assignment.Target, value)
+                .SetSymbolValue(assignment.Target.TrackedSymbol(state), value);
+        }
+        else
+        {
+            return null;
+        }
+    }
+
     private static ProgramState ProcessCompoundAssignment(ProgramState state, ICompoundAssignmentOperationWrapper assignment)
     {
         if ((state.HasConstraint(assignment.Target, ObjectConstraint.NotNull) && state.HasConstraint(assignment.Value, ObjectConstraint.NotNull))

analyzers/tests/SonarAnalyzer.UnitTest/TestCases/SymbolicExecution/Roslyn/ConditionEvaluatesToConstant.CSharp8.cs

@@ -496,21 +496,6 @@ int SwitchExpression()
     }
 }
 
-// https://github.com/SonarSource/sonar-dotnet/issues/8094
-class Repro8094
-{
-    public void TestMethod()
-    {
-        Action? someDelegate = delegate { };
-        someDelegate += Callback;
-        someDelegate -= Callback;
-        if (someDelegate == null) // Noncompliant {{Change this condition so that it does not always evaluate to 'False'.}}
-        { }
-    }
-
-    private void Callback() { }
-}
-
 // https://github.com/SonarSource/sonar-dotnet/issues/8149
 class Repro_8149
 {

analyzers/tests/SonarAnalyzer.UnitTest/TestCases/SymbolicExecution/Roslyn/ConditionEvaluatesToConstant.cs

@@ -3483,3 +3483,36 @@ public static void Foo()
         }
     }
 }
+
+// Reproducer for https://github.com/SonarSource/sonar-dotnet/issues/8094
+public class Repro_8094
+{
+    public Action this[int index] { get => null; set { _ = value; } }
+
+    public void TestMethod()
+    {
+        Action someDelegate = delegate { };
+        someDelegate += Callback;
+        someDelegate -= Callback;
+
+        if (someDelegate == null) // Compliant
+        {
+            Console.WriteLine();
+        }
+
+        var delegateCopy = someDelegate -= Callback;
+        if (delegateCopy == null) // Compliant
+        {
+            Console.WriteLine();
+        }
+
+        this[42] += Callback;
+        this[42] -= Callback;
+        if (this[42] == null)     // Compliant
+        {
+            Console.WriteLine();
+        }
+    }
+
+    private void Callback() { }
+}