Fix RCS1258 for partially shared flags enums

ChangeLog.md

@@ -7,6 +7,10 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 
 ## [Unreleased]
 
+### Fixed
+
+- Fix enum contained flags check for partial matches in [RCS1258](https://josefpihrt.github.io/docs/roslynator/analyzers/RCS1258) ([PR](https://github.com/dotnet/roslynator/pull/1740) by @ovska)
+
 ## [4.15.0] - 2025-12-14
 
 ### Added

src/Analyzers/CSharp/Analysis/UnnecessaryEnumFlagAnalyzer.cs

@@ -62,8 +62,9 @@ private static void AnalyzeBitwiseOrExpression(SyntaxNodeAnalysisContext context
                 for (int i = values.Count - 1; i >= 0; i--)
                 {
                     (ExpressionSyntax expression2, ulong value2) = values[i];
+                    ulong sharedBits = value & value2;
 
-                    if ((value & value2) != 0)
+                    if (sharedBits == value2 || sharedBits == value)
                     {
                         if (value <= value2)
                         {

src/Tests/Analyzers.Tests/RCS1258UnnecessaryEnumFlagTests.cs

@@ -79,4 +79,28 @@ void M()
 }
 ");
     }
+
+    [Fact, Trait(Traits.Analyzer, DiagnosticIdentifiers.UnnecessaryEnumFlag)]
+    public async Task TestNoDiagnostic_PartiallySame()
+    {
+        await VerifyNoDiagnosticAsync(
+            @"
+[System.Flags]
+enum FooBar
+{
+    Shared = 1,
+    Foo = Shared | 2,
+    Bar = Shared | 4,
+}
+
+class C
+{
+    public static bool M(FooBar x)
+    {
+        return x == (FooBar.Foo | FooBar.Bar);
+    }
+}
+"
+        );
+    }
 }