Fix anonymous-type lambda early-return emitting unresolvable cast

When a lambda's inferred return type contains an anonymous type and one
branch returns null, the decompiler emitted an explicit cast such as
`return (IEnumerable<<>f__AnonymousType0<int>>)null;`, which is invalid C#.

Skip the cast in IsPossibleLossOfTypeInformation for null literals whenever
the expected type contains an anonymous type:
null is implicitly convertible to any reference type, so no cast is needed,
and the anonymous type has no nameable form to cast to anyway.

Fixes #3751

Author: jakobhellermann

ICSharpCode.Decompiler.Tests/PrettyTestRunner.cs

@@ -936,6 +936,12 @@ public async Task Issue3684([ValueSource(nameof(roslyn4OrNewerOptions))] Compile
 			await RunForLibrary(cscOptions: cscOptions);
 		}
 
+		[Test]
+		public async Task Issue3751([ValueSource(nameof(defaultOptions))] CompilerOptions cscOptions)
+		{
+			await RunForLibrary(cscOptions: cscOptions);
+		}
+
 		async Task RunForLibrary([CallerMemberName] string testName = null, AssemblerOptions asmOptions = AssemblerOptions.None, CompilerOptions cscOptions = CompilerOptions.None, Action<DecompilerSettings> configureDecompiler = null)
 		{
 			await Run(testName, asmOptions | AssemblerOptions.Library, cscOptions | CompilerOptions.Library, configureDecompiler);

ICSharpCode.Decompiler.Tests/TestCases/Pretty/Issue3751.cs

@@ -0,0 +1,28 @@
+using System;
+
+namespace ICSharpCode.Decompiler.Tests.TestCases.Pretty
+{
+	internal class Issue3751
+	{
+		private static bool Cond;
+
+		private static T Infer<T>(Func<T> factory)
+		{
+			return factory();
+		}
+
+		public object Trigger()
+		{
+			return Infer(delegate {
+				if (Cond)
+				{
+					Console.WriteLine();
+					return null;
+				}
+				return new {
+					Value = 1
+				};
+			});
+		}
+	}
+}

ICSharpCode.Decompiler/CSharp/StatementBuilder.cs

@@ -412,7 +412,7 @@ private bool IsPossibleLossOfTypeInformation(IType givenType, IType expectedType
 			if (expectedType == SpecialType.Dynamic)
 				return true;
 			if (givenType == SpecialType.NullType)
-				return true;
+				return !expectedType.ContainsAnonymousType();
 			return false;
 		}