[Pretty Tests] add two tests for delegate caching

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -332,6 +332,12 @@ namespace ICSharpCode.Decompiler.Tests
 			});
 		}
 
+		[Test]
+		public async Task DelegateCaching([ValueSource(nameof(roslyn4OrNewerOptions))] CompilerOptions cscOptions)
+		{
+			await RunForLibrary(cscOptions: cscOptions);
+		}
+
 		[Test]
 		public async Task AnonymousTypes([ValueSource(nameof(defaultOptionsWithMcs))] CompilerOptions cscOptions)
 		{

ICSharpCode.Decompiler.Tests/TestCases/Pretty/DelegateCaching.cs

@@ -0,0 +1,40 @@
+// Copyright (c) 2026 marcusmalloc
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+using System;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
+{
+	internal static class DelegateCaching
+	{
+		// Issue #3921: explicit construction must preserve a fresh delegate on each call.
+		public static Action FreshDelegate()
+		{
+			return new Action(M);
+		}
+
+		public static Action CachedDelegate()
+		{
+			return M;
+		}
+
+		private static void M()
+		{
+		}
+	}
+}