Fix lowering of taghelpers inside a section

The IR lowering phase was attaching the 'tag helper fields' node to the
builder instead of to the top-level node (document). This meant that
things wouldn't be where we expect when the first tag helper occurrence is
inside a directive block (section).

Found this porting MVC to use the new Razor codebase.

src/Microsoft.AspNetCore.Razor.Evolution/DefaultRazorIRLoweringPhase.cs

@@ -17,6 +17,7 @@ namespace Microsoft.AspNetCore.Razor.Evolution
             ThrowForMissingDependency(syntaxTree);
 
             var builder = RazorIRBuilder.Document();
+            var document = (DocumentIRNode)builder.Current;
             var namespaces = new HashSet<string>();
 
             var i = 0;
@@ -40,7 +41,7 @@ namespace Microsoft.AspNetCore.Razor.Evolution
             var imports = codeDocument.GetImportSyntaxTrees();
             if (imports != null)
             {
-                var importsVisitor = new ImportsVisitor(builder, namespaces);
+                var importsVisitor = new ImportsVisitor(document, builder, namespaces);
 
                 for (var j = 0; j < imports.Count; j++)
                 {
@@ -51,24 +52,25 @@ namespace Microsoft.AspNetCore.Razor.Evolution
                 }
             }
 
-            var visitor = new MainSourceVisitor(builder, namespaces)
+            var visitor = new MainSourceVisitor(document, builder, namespaces)
             {
                  Filename = syntaxTree.Source.Filename,
             };
 
             visitor.VisitBlock(syntaxTree.Root);
-
+            
-            var irDocument = (DocumentIRNode)builder.Build();
+            codeDocument.SetIRDocument(document);
-            codeDocument.SetIRDocument(irDocument);
         }
 
         private class LoweringVisitor : ParserVisitor
         {
             protected readonly RazorIRBuilder _builder;
+            protected readonly DocumentIRNode _document;
             protected readonly HashSet<string> _namespaces;
 
-            public LoweringVisitor(RazorIRBuilder builder, HashSet<string> namespaces)
+            public LoweringVisitor(DocumentIRNode document, RazorIRBuilder builder, HashSet<string> namespaces)
             {
+                _document = document;
                 _builder = builder;
                 _namespaces = namespaces;
             }
@@ -146,8 +148,8 @@ namespace Microsoft.AspNetCore.Razor.Evolution
             // this simple.
             private bool _insideLineDirective;
 
-            public ImportsVisitor(RazorIRBuilder builder, HashSet<string> namespaces)
+            public ImportsVisitor(DocumentIRNode document, RazorIRBuilder builder, HashSet<string> namespaces)
-                : base(builder, namespaces)
+                : base(document, builder, namespaces)
             {
             }
 
@@ -182,8 +184,8 @@ namespace Microsoft.AspNetCore.Razor.Evolution
         {
             private DeclareTagHelperFieldsIRNode _tagHelperFields;
 
-            public MainSourceVisitor(RazorIRBuilder builder, HashSet<string> namespaces)
+            public MainSourceVisitor(DocumentIRNode document, RazorIRBuilder builder, HashSet<string> namespaces)
-                : base(builder, namespaces)
+                : base(document, builder, namespaces)
             {
             }
 
@@ -416,8 +418,8 @@ namespace Microsoft.AspNetCore.Razor.Evolution
             {
                 if (_tagHelperFields == null)
                 {
-                    _tagHelperFields = new DeclareTagHelperFieldsIRNode();
+                    _tagHelperFields = new DeclareTagHelperFieldsIRNode() { Parent = _document, };
-                    _builder.Add(_tagHelperFields);
+                    _document.Children.Add(_tagHelperFields);
                 }
 
                 foreach (var descriptor in block.Descriptors)

test/Microsoft.AspNetCore.Razor.Evolution.Test/Intermediate/DefaultRazorIRLoweringPhaseIntegrationTest.cs

@@ -194,6 +194,57 @@ namespace Microsoft.AspNetCore.Razor.Evolution.Intermediate
                 });
         }
 
+        [Fact]
+        public void Lower_TagHelper_InSection()
+        {
+            // Arrange
+            var codeDocument = TestRazorCodeDocument.Create(@"@addTagHelper *, TestAssembly
+@section test {
+<span val=""@Hello World""></span>
+}");
+            var tagHelpers = new[]
+            {
+                new TagHelperDescriptor
+                {
+                    TagName = "span",
+                    TypeName = "SpanTagHelper",
+                    AssemblyName = "TestAssembly",
+                }
+            };
+
+            // Act
+            var irDocument = Lower(codeDocument, tagHelpers: tagHelpers);
+
+            // Assert
+            Children(
+                irDocument,
+                n => Checksum(n),
+                n => Using("System", n),
+                n => Using(typeof(Task).Namespace, n),
+                n => Directive(
+                    "section",
+                    n,
+                    c1 => DirectiveToken(DirectiveTokenKind.Member, "test", c1),
+                    c1 => Html(Environment.NewLine, c1),
+                    c1 => 
+                    {
+                        var tagHelperNode = Assert.IsType<TagHelperIRNode>(c1);
+                        Children(
+                            tagHelperNode,
+                            c2 => TagHelperStructure("span", TagMode.StartTagAndEndTag, c2),
+                            c2 => Assert.IsType<CreateTagHelperIRNode>(c2),
+                            c2 => TagHelperHtmlAttribute(
+                                "val",
+                                HtmlAttributeValueStyle.DoubleQuotes,
+                                c2,
+                                v => CSharpAttributeValue(string.Empty, "Hello", v),
+                                v => LiteralAttributeValue(" ", "World", v)),
+                            c2 => Assert.IsType<ExecuteTagHelpersIRNode>(c2));
+                    },
+                    c1 => Html(Environment.NewLine, c1)),
+                n => TagHelperFieldDeclaration(n, "SpanTagHelper"));
+        }
+
         [Fact]
         public void Lower_TagHelpersWithBoundAttribute()
         {