Code fix for RCS1197 should add parentheses if necessary

ChangeLog.md

@@ -25,6 +25,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 - [CLI] Fix filtering of projects (relates to `--projects` or `--ignored-projects` parameter) ([#914](https://github.com/josefpihrt/roslynator/pull/914)).
 - Refactoring "Add using directive" (RR0014) now works when file-scoped namespace is used ([#932](https://github.com/josefpihrt/roslynator/pull/932)).
+- Add parentheses if necessary in a code fix for [RCS1197](https://github.com/JosefPihrt/Roslynator/blob/main/docs/analyzers/RCS1197.md) ([#928](https://github.com/josefpihrt/roslynator/pull/928) by @karl-sjogren).
 
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 <!-- Content below does not adhere to 'Keep a Changelog' format -->

src/Analyzers.CodeFixes/CSharp/CodeFixes/OptimizeStringBuilderAppendCallCodeFixProvider.cs

@@ -264,7 +264,7 @@ private static InvocationExpressionSyntax CreateInvocationExpression(
                                         Argument(
                                             SubtractExpression(
                                                 SimpleMemberAccessExpression(invocationInfo.Expression, IdentifierName("Length")),
-                                                arguments[0].Expression)));
+                                                arguments[0].Expression.Parenthesize())));
 
                                     return CreateNewInvocationExpression(outerInvocationExpression, "Append", argumentList);
                                 }

src/Tests/Analyzers.Tests/RCS1197OptimizeStringBuilderAppendCallTests.cs

@@ -44,6 +44,42 @@ void M()
 ");
         }
 
+        [Fact, Trait(Traits.Analyzer, DiagnosticIdentifiers.OptimizeStringBuilderAppendCall)]
+        public async Task Test_Substring_Int32_Int32_Calculation()
+        {
+            await VerifyDiagnosticAndFixAsync(@"
+using System.Text;
+
+class C
+{
+    void M()
+    {
+        string s = null;
+        var start = 2;
+        var len = 5;
+        var sb = new StringBuilder();
+
+        sb.Append([|s.Substring(start + len)|]);
+    }
+}
+", @"
+using System.Text;
+
+class C
+{
+    void M()
+    {
+        string s = null;
+        var start = 2;
+        var len = 5;
+        var sb = new StringBuilder();
+
+        sb.Append(s, start + len, s.Length - (start + len));
+    }
+}
+");
+        }
+
         [Fact, Trait(Traits.Analyzer, DiagnosticIdentifiers.OptimizeStringBuilderAppendCall)]
         public async Task Test_Substring_Int32()
         {