Fix #4016: Don't consume nullable flags for anonymous type arguments

Roslyn writes [Nullable] for an anonymous type as if the type was not
generic, so its type arguments have no flags. Taking one per argument
shifted every annotation after the anonymous type, which showed on the
cached lambda fields in <>c, and tripped the dummy-flag assert when the
shifted stream landed on a generic struct. [Dynamic] and
[TupleElementNames] do have entries for those arguments (checked against
what Roslyn emits), so only the nullable index skips them.

ICSharpCode.Decompiler.Tests/IsolatedMethodDecompilationTests.cs

@@ -86,6 +86,40 @@ namespace ICSharpCode.Decompiler.Tests
 			await DecompileNestedGetMethod("BlockExtensions").ConfigureAwait(false);
 		}
 
+		[Test]
+		public async Task AnonymousTypeArgumentsHaveNoNullableFlags()
+		{
+			// #4016: NullableAttribute has no entries for the type arguments of an anonymous type,
+			// so the annotations after it must not shift. The affected fields live in the lambda
+			// cache class, which only shows up when it is decompiled on its own.
+			var csFile = Path.Combine(TestCasePath, "AnonymousTypeNullability.cs");
+			var compilation = await Tester.CompileCSharp(csFile, CompilerOptions.UseRoslynLatest | CompilerOptions.Optimize | CompilerOptions.Library | CompilerOptions.NullableEnable).ConfigureAwait(false);
+			try
+			{
+				var settings = new DecompilerSettings();
+				using var file = new FileStream(compilation.PathToAssembly, FileMode.Open, FileAccess.Read);
+				var module = new PEFile(compilation.PathToAssembly, file, PEStreamOptions.PrefetchEntireImage);
+				var targetFramework = module.Metadata.DetectTargetFrameworkId();
+				var resolver = new UniversalAssemblyResolver(compilation.PathToAssembly, false, targetFramework, null, PEStreamOptions.PrefetchMetadata);
+				resolver.AddSearchDirectory(Tester.RefAssembliesToolset.GetPath(targetFramework));
+				var typeSystem = new DecompilerTypeSystem(module, resolver, settings);
+				var decompiler = new CSharpDecompiler(typeSystem, settings);
+
+				var lambdaCache = typeSystem.MainModule.TypeDefinitions
+					.Single(t => t.Name == "AnonymousTypeNullability")
+					.NestedTypes.Single(t => t.Name == "<>c");
+
+				var code = decompiler.Decompile(lambdaCache.MetadataToken).ToString();
+
+				Assert.That(code, Does.Contain("Func<<>f__AnonymousType0<string, string>, List<string?>> <>9__0_1;"));
+				Assert.That(code, Does.Contain("Func<<>f__AnonymousType1<string>, KeyValuePair<string, string?>> <>9__1_1;"));
+			}
+			finally
+			{
+				compilation.DeleteTempFiles();
+			}
+		}
+
 		static async Task DecompileNestedGetMethod(string containerName)
 		{
 			var csFile = Path.Combine(TestCasePath, "ExtensionContainerNestedType.cs");