fix(analyzers): skip RCS1231 for sync Task-returning methods

ChangeLog.md

@@ -13,6 +13,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Fix analyzer [RCS1265](https://josefpihrt.github.io/docs/roslynator/analyzers/RCS1265) to not report catch clauses with a `when` filter ([PR](https://github.com/dotnet/roslynator/pull/1789))
 - Fix analyzer [RCS0034](https://josefpihrt.github.io/docs/roslynator/analyzers/RCS0034) for types with a primary constructor and multiple constraint clauses ([PR](https://github.com/dotnet/roslynator/pull/1791))
 - [CLI] Fix GitLab output format to use relative paths, forward slashes, and 1-based line numbers ([PR](https://github.com/dotnet/roslynator/pull/1792))
+- Fix analyzer [RCS1231](https://josefpihrt.github.io/docs/roslynator/analyzers/RCS1231) to not report `CancellationToken` in sync methods returning `Task` ([PR](https://github.com/dotnet/roslynator/pull/1802))
 
 ## [4.16.0] - 2026-08-08
 

src/Analyzers/CSharp/Analysis/RefReadOnlyParameterAnalyzer.cs

@@ -142,6 +142,12 @@ private static void Analyze(
                 continue;
             }
 
+            if (type.HasMetadataName(in MetadataNames.System_Threading_CancellationToken)
+                && methodSymbol.ReturnType.IsWellKnownTaskType())
+            {
+                continue;
+            }
+
             if (parameter.RefKind != RefKind.None)
                 continue;
 

src/Tests/Analyzers.Tests/RCS1231MakeParameterRefReadOnlyTests.cs

@@ -215,6 +215,49 @@ public void M(DateTime dt)
 
     public DateTime P { get; set; }
 }
+");
+    }
+
+    [Fact, Trait(Traits.Analyzer, DiagnosticIdentifiers.MakeParameterRefReadOnly)]
+    public async Task TestNoDiagnostic_CancellationToken_SyncTaskReturningMethod()
+    {
+        await VerifyNoDiagnosticAsync(@"
+using System.Threading;
+using System.Threading.Tasks;
+
+class C
+{
+    public Task DoThatWay(CancellationToken cancellationToken)
+    {
+        return Task.FromCanceled(cancellationToken);
+    }
+}
+");
+    }
+
+    [Fact, Trait(Traits.Analyzer, DiagnosticIdentifiers.MakeParameterRefReadOnly)]
+    public async Task Test_ReadOnlyStruct_SyncTaskReturningMethod()
+    {
+        await VerifyDiagnosticAndFixAsync(@"
+using System.Threading.Tasks;
+
+readonly struct C
+{
+    public Task<int> M(C [|c|])
+    {
+        return Task.FromResult(c.GetHashCode());
+    }
+}
+", @"
+using System.Threading.Tasks;
+
+readonly struct C
+{
+    public Task<int> M(in C c)
+    {
+        return Task.FromResult(c.GetHashCode());
+    }
+}
 ");
     }
 }