Fix: Support compound assignments in "Use null propagation" analyzer (#81328)

Author: CyrusNajmabadi

src/Analyzers/CSharp/Tests/UseNullPropagation/UseNullPropagationTests.cs

@@ -2913,4 +2913,115 @@ public void Dispose()
                 }
             }
             """);
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/81322")]
+    public Task TestIfStatement_CompoundAssignment_Event()
+        => TestInRegularAndScriptAsync(
+            """
+            using System;
+
+            class C
+            {
+                event Action SomeEvent;
+
+                static void M(C c)
+                {
+                    [|if|] (c is not null)
+                        c.SomeEvent += () => { };
+                }
+            }
+            """,
+            """
+            using System;
+
+            class C
+            {
+                event Action SomeEvent;
+
+                static void M(C c)
+                {
+                    c?.SomeEvent += () => { };
+                }
+            }
+            """, languageVersion: LanguageVersion.CSharp14);
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/81322")]
+    public Task TestIfStatement_CompoundAssignment_AddAssignment()
+        => TestInRegularAndScriptAsync(
+            """
+            using System;
+
+            class C
+            {
+                int Value;
+
+                static void M(C c)
+                {
+                    [|if|] (c != null)
+                        c.Value += 5;
+                }
+            }
+            """,
+            """
+            using System;
+
+            class C
+            {
+                int Value;
+
+                static void M(C c)
+                {
+                    c?.Value += 5;
+                }
+            }
+            """, languageVersion: LanguageVersion.CSharp14);
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/81322")]
+    public Task TestIfStatement_CompoundAssignment_SubtractAssignment()
+        => TestInRegularAndScriptAsync(
+            """
+            using System;
+
+            class C
+            {
+                int Value;
+
+                static void M(C c)
+                {
+                    [|if|] (c != null)
+                        c.Value -= 5;
+                }
+            }
+            """,
+            """
+            using System;
+
+            class C
+            {
+                int Value;
+
+                static void M(C c)
+                {
+                    c?.Value -= 5;
+                }
+            }
+            """, languageVersion: LanguageVersion.CSharp14);
+
+    [Fact, WorkItem("https://github.com/dotnet/roslyn/issues/81322")]
+    public Task TestIfStatement_CompoundAssignment_NotAvailableInCSharp13()
+        => TestMissingInRegularAndScriptAsync(
+            """
+            using System;
+
+            class C
+            {
+                event Action SomeEvent;
+
+                static void M(C c)
+                {
+                    if (c is not null)
+                        c.SomeEvent += () => { };
+                }
+            }
+            """, LanguageVersion.CSharp13);
 }

src/Analyzers/Core/Analyzers/UseCoalesceExpression/AbstractUseCoalesceExpressionForIfNullCheckDiagnosticAnalyzer.cs

@@ -96,7 +96,7 @@ private void AnalyzeSyntax(SyntaxNodeAnalysisContext context)
             if (!AnalyzeLocalDeclarationForm(previousStatement, out expressionToCoalesce))
                 return;
         }
-        else if (syntaxFacts.IsAnyAssignmentStatement(previousStatement))
+        else if (syntaxFacts.IsSimpleAssignmentStatement(previousStatement))
         {
             // v = Expr();
             // if (v == null)

src/Analyzers/Core/Analyzers/UseNullPropagation/AbstractUseNullPropagationDiagnosticAnalyzer.cs

@@ -440,7 +440,8 @@ private static TExpressionSyntax RemoveObjectCastIfAny(
         if (node is TElementAccessExpressionSyntax elementAccess)
             return (TExpressionSyntax?)syntaxFacts.GetExpressionOfElementAccessExpression(elementAccess);
 
-        if (syntaxFacts.SyntaxKinds.SimpleAssignmentExpression == node.RawKind && syntaxFacts.SupportsNullConditionalAssignment(node.SyntaxTree.Options))
+        if (syntaxFacts.IsAnyAssignmentStatement(node.Parent) &&
+            syntaxFacts.SupportsNullConditionalAssignment(node.SyntaxTree.Options))
         {
             syntaxFacts.GetPartsOfAssignmentExpressionOrStatement(node, out var left, out _, out _);
             return (TExpressionSyntax)left;

src/Analyzers/Core/Analyzers/UseNullPropagation/AbstractUseNullPropagationDiagnosticAnalyzer_IfStatement.cs

@@ -94,7 +94,7 @@ private void AnalyzeIfStatementAndReportDiagnostic(
             // `if (<expr> != null) { <expr>.Method(); <expr> = null; }`
             //
             // If 'expr' is not null, then we execute the body and then end up with expr being null.  So `expr?.Method(); expr = null;`
-            // preserves those semantics.  Simialrly, if is expr is null, then `expr?.Method();` does nothing, and `expr = null` keeps it
+            // preserves those semantics.  Simialarly, if is expr is null, then `expr?.Method();` does nothing, and `expr = null` keeps it
             // the same as well.  So this is a valid conversion in all cases.
             if (!syntaxFacts.IsSimpleAssignmentStatement(nullAssignmentOpt))
                 return null;

src/Workspaces/SharedUtilitiesAndExtensions/Compiler/CSharp/Services/SyntaxFacts/CSharpSyntaxFacts.cs

@@ -1203,9 +1203,11 @@ public bool IsDeclaration(SyntaxNode? node)
     public bool IsTypeDeclaration(SyntaxNode node)
         => SyntaxFacts.IsTypeDeclaration(node.Kind());
 
-    public bool IsSimpleAssignmentStatement([NotNullWhen(true)] SyntaxNode? statement)
-        => statement is ExpressionStatementSyntax exprStatement &&
-           exprStatement.Expression.IsKind(SyntaxKind.SimpleAssignmentExpression);
+    public bool IsSimpleAssignmentStatement([NotNullWhen(true)] SyntaxNode? node)
+        => node is ExpressionStatementSyntax { Expression: (kind: SyntaxKind.SimpleAssignmentExpression) };
+
+    public bool IsAnyAssignmentStatement([NotNullWhen(true)] SyntaxNode? node)
+        => node is ExpressionStatementSyntax { Expression: AssignmentExpressionSyntax };
 
     public void GetPartsOfAssignmentStatement(
         SyntaxNode statement, out SyntaxNode left, out SyntaxToken operatorToken, out SyntaxNode right)
@@ -1229,10 +1231,6 @@ public void GetPartsOfAssignmentExpressionOrStatement(
         right = assignment.Right;
     }
 
-    // C# does not have assignment statements.
-    public bool IsAnyAssignmentStatement([NotNullWhen(true)] SyntaxNode? node)
-        => false;
-
     public SyntaxToken GetIdentifierOfSimpleName(SyntaxNode node)
         => ((SimpleNameSyntax)node).Identifier;