Add test for nested type

src/EditorFeatures/Test/MetadataAsSource/MetadataAsSourceTests.cs

@@ -2970,6 +2970,80 @@ public class TestType
             context.VerifyResult(metadataAsSourceFile, expected);
         }
 
+        [Fact, Trait(Traits.Feature, Traits.Features.MetadataAsSource)]
+        public async Task TestNullableEnableDisable13()
+        {
+            var metadata = @"
+#nullable enable
+
+using System;
+
+public class TestType
+{
+    public void M1(string s)
+    {
+    }
+
+#nullable disable
+
+    public class Nested
+    {
+        public void NestedM(string s)
+        {
+        }
+    }
+
+#nullable enable
+
+    public void M2(string s)
+    {
+    }
+}";
+            var sourceWithSymbolReference = @"
+class C
+{
+    void M()
+    {
+        var obj = new TestType().[|M1|](null);
+    }
+}";
+            var expected = $@"#region {FeaturesResources.Assembly} ReferencedAssembly, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null
+// {CodeAnalysisResources.InMemoryAssembly}
+#endregion
+
+#nullable enable
+
+public class TestType
+{{
+    public TestType();
+
+    public void [|M1|](string s);
+    public void M2(string s);
+
+    public class Nested
+    {{
+        public Nested();
+
+#nullable disable
+        public void NestedM(string s);
+
+#nullable enable
+    }}                    
+}}";
+
+            using var context = TestContext.Create(
+                LanguageNames.CSharp,
+                SpecializedCollections.SingletonEnumerable(metadata),
+                includeXmlDocComments: false,
+                languageVersion: "CSharp8",
+                sourceWithSymbolReference: sourceWithSymbolReference,
+                metadataLanguageVersion: "CSharp8");
+
+            var navigationSymbol = await context.GetNavigationSymbolAsync();
+            var metadataAsSourceFile = await context.GenerateSourceAsync(navigationSymbol);
+            context.VerifyResult(metadataAsSourceFile, expected);
+        }
+
         [Fact, Trait(Traits.Feature, Traits.Features.MetadataAsSource)]
         public async Task TestDynamic1()
         {

src/Features/CSharp/Portable/MetadataAsSource/CSharpMetadataAsSourceService.cs

@@ -143,13 +143,14 @@ private static SyntaxTrivia[] CreateNullableTrivia(bool enable)
             };
         }
 
-        private SyntaxNode AddNullableRegions(SyntaxNode node, CancellationToken cancellationToken)
+        private TSyntax AddNullableRegions<TSyntax>(TSyntax node, CancellationToken cancellationToken)
+            where TSyntax : SyntaxNode
         {
             return node switch
             {
-                CompilationUnitSyntax compilationUnit => compilationUnit.WithMembers(AddNullableRegions(compilationUnit.Members, cancellationToken)),
-                NamespaceDeclarationSyntax ns => ns.WithMembers(AddNullableRegions(ns.Members, cancellationToken)),
-                TypeDeclarationSyntax type => AddNullableRegionsAroundTypeMembers(type, cancellationToken),
+                CompilationUnitSyntax compilationUnit => (TSyntax)(object)compilationUnit.WithMembers(AddNullableRegions(compilationUnit.Members, cancellationToken)),
+                NamespaceDeclarationSyntax ns => (TSyntax)(object)ns.WithMembers(AddNullableRegions(ns.Members, cancellationToken)),
+                TypeDeclarationSyntax type => (TSyntax)(object)AddNullableRegionsAroundTypeMembers(type, cancellationToken),
                 _ => node,
             };
         }
@@ -161,7 +162,7 @@ private SyntaxNode AddNullableRegions(SyntaxNode node, CancellationToken cancell
             using var _ = ArrayBuilder<MemberDeclarationSyntax>.GetInstance(out var builder);
 
             foreach (var member in members)
-                builder.Add((MemberDeclarationSyntax)AddNullableRegions(member, cancellationToken));
+                builder.Add(AddNullableRegions(member, cancellationToken));
 
             return SyntaxFactory.List(builder);
         }
@@ -181,7 +182,7 @@ private SyntaxNode AddNullableRegions(SyntaxNode node, CancellationToken cancell
                 {
                     // if we hit a type, and we're currently disabled, then switch us back to enabled for that type.
                     // This ensures whenever we walk into a type-decl, we're always in the enabled-state.
-                    builder.Add(TransitionTo(member, enabled: true, ref currentlyEnabled));
+                    builder.Add(TransitionTo(AddNullableRegions(member, cancellationToken), enabled: true, ref currentlyEnabled));
                     continue;
                 }