Fix: Nested tag helpers do not work when tag helper prefix is set

src/Microsoft.AspNetCore.Razor.Language/Legacy/TagHelperParseTreeRewriter.cs

@@ -44,7 +44,6 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
         private readonly Stack<BlockBuilder> _blockStack;
         private TagHelperBlockTracker _currentTagHelperTracker;
         private BlockBuilder _currentBlock;
-        private string _currentParentTagName;
 
         public TagHelperParseTreeRewriter(string tagHelperPrefix, IEnumerable<TagHelperDescriptor> descriptors)
         {
@@ -56,6 +55,12 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
             _htmlAttributeTracker = new List<KeyValuePair<string, string>>();
         }
 
+        private TagBlockTracker CurrentTracker => _trackerStack.Count > 0 ? _trackerStack.Peek() : null;
+
+        private string CurrentParentTagName => CurrentTracker?.TagName;
+
+        private bool CurrentParentIsTagHelper => CurrentTracker?.IsTagHelper ?? false;
+
         public Block Rewrite(Block syntaxTree, ErrorSink errorSink)
         {
             RewriteTags(syntaxTree, errorSink, depth: 0);
@@ -189,7 +194,11 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
                 // We're now in a start tag block, we first need to see if the tag block is a tag helper.
                 var elementAttributes = GetAttributeNameValuePairs(tagBlock);
 
-                tagHelperBinding = _tagHelperBinder.GetBinding(tagName, elementAttributes, _currentParentTagName);
+                tagHelperBinding = _tagHelperBinder.GetBinding(
+                    tagName,
+                    elementAttributes,
+                    CurrentParentTagName,
+                    CurrentParentIsTagHelper);
 
                 // If there aren't any TagHelperDescriptors registered then we aren't a TagHelper
                 if (tagHelperBinding == null)
@@ -257,7 +266,8 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
                     tagHelperBinding = _tagHelperBinder.GetBinding(
                         tagName,
                         attributes: Array.Empty<KeyValuePair<string, string>>(),
-                        parentTagName: _currentParentTagName);
+                        parentTagName: CurrentParentTagName,
+                        parentIsTagHelper: CurrentParentIsTagHelper);
 
                     // If there are not TagHelperDescriptors associated with the end tag block that also have no
                     // required attributes then it means we can't be a TagHelper, bail out.
@@ -508,10 +518,20 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
                 return;
             }
 
-            var currentTracker = _trackerStack.Count > 0 ? _trackerStack.Peek() : null;
+            if (!HasAllowedChildren())
+            {
+                return;
+            }
 
-            if (HasAllowedChildren() &&
+            var tagHelperBinding = _tagHelperBinder.GetBinding(
-                !_currentTagHelperTracker.AllowedChildren.Contains(tagName, StringComparer.OrdinalIgnoreCase))
+                tagName,
+                attributes: Array.Empty<KeyValuePair<string, string>>(),
+                parentTagName: CurrentParentTagName,
+                parentIsTagHelper: CurrentParentIsTagHelper);
+
+            // If we found a binding for the current tag, then it is a tag helper. Use the prefixed allowed children to compare.
+            var allowedChildren = tagHelperBinding != null ? _currentTagHelperTracker.PrefixedAllowedChildren : _currentTagHelperTracker.AllowedChildren;
+            if (!allowedChildren.Contains(tagName, StringComparer.OrdinalIgnoreCase))
             {
                 OnAllowedChildrenTagError(_currentTagHelperTracker, tagName, tagBlock, errorSink);
             }
@@ -812,14 +832,12 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
 
         private void PushTrackerStack(TagBlockTracker tracker)
         {
-            _currentParentTagName = tracker.TagName;
             _trackerStack.Push(tracker);
         }
 
         private TagBlockTracker PopTrackerStack()
         {
             var poppedTracker = _trackerStack.Pop();
-            _currentParentTagName = _trackerStack.Count > 0 ? _trackerStack.Peek().TagName : null;
 
             return poppedTracker;
         }

src/Microsoft.AspNetCore.Razor.Language/TagHelperBinder.cs

@@ -39,12 +39,14 @@ namespace Microsoft.AspNetCore.Razor.Language
         /// retrieves catch-all <see cref="TagHelperDescriptor"/>s (descriptors that target every tag).</param>
         /// <param name="attributes">Attributes on the HTML tag.</param>
         /// <param name="parentTagName">The parent tag name of the given <paramref name="tagName"/> tag.</param>
+        /// <param name="parentIsTagHelper">Is the parent tag of the given <paramref name="tagName"/> tag a tag helper.</param>
         /// <returns><see cref="TagHelperDescriptor"/>s that apply to the given HTML tag criteria.
         /// Will return <c>null</c> if no <see cref="TagHelperDescriptor"/>s are a match.</returns>
         public TagHelperBinding GetBinding(
             string tagName,
             IReadOnlyList<KeyValuePair<string, string>> attributes,
-            string parentTagName)
+            string parentTagName,
+            bool parentIsTagHelper)
         {
             if (!string.IsNullOrEmpty(_tagHelperPrefix) &&
                 (tagName.Length <= _tagHelperPrefix.Length ||
@@ -74,11 +76,17 @@ namespace Microsoft.AspNetCore.Razor.Language
             }
 
             var tagNameWithoutPrefix = _tagHelperPrefix != null ? tagName.Substring(_tagHelperPrefix.Length) : tagName;
+            var parentTagNameWithoutPrefix = parentTagName;
+            if (_tagHelperPrefix != null && parentIsTagHelper)
+            {
+                parentTagNameWithoutPrefix = parentTagName.Substring(_tagHelperPrefix.Length);
+            }
+
             Dictionary<TagHelperDescriptor, IReadOnlyList<TagMatchingRuleDescriptor>> applicableDescriptorMappings = null;
             foreach (var descriptor in descriptors)
             {
                 var applicableRules = descriptor.TagMatchingRules.Where(
-                    rule => TagHelperMatchingConventions.SatisfiesRule(tagNameWithoutPrefix, parentTagName, attributes, rule));
+                    rule => TagHelperMatchingConventions.SatisfiesRule(tagNameWithoutPrefix, parentTagNameWithoutPrefix, attributes, rule));
 
                 if (applicableRules.Any())
                 {

src/Microsoft.AspNetCore.Razor.Language/TagHelperMatchingConventions.cs

@@ -16,7 +16,7 @@ namespace Microsoft.AspNetCore.Razor.Language
 
         public static bool SatisfiesRule(
             string tagNameWithoutPrefix,
-            string parentTagName,
+            string parentTagNameWithoutPrefix,
             IEnumerable<KeyValuePair<string, string>> tagAttributes,
             TagMatchingRuleDescriptor rule)
         {
@@ -41,7 +41,7 @@ namespace Microsoft.AspNetCore.Razor.Language
                 return false;
             }
 
-            var satisfiesParentTag = SatisfiesParentTag(parentTagName, rule);
+            var satisfiesParentTag = SatisfiesParentTag(parentTagNameWithoutPrefix, rule);
             if (!satisfiesParentTag)
             {
                 return false;
@@ -89,14 +89,14 @@ namespace Microsoft.AspNetCore.Razor.Language
             return true;
         }
 
-        public static bool SatisfiesParentTag(string parentTagName, TagMatchingRuleDescriptor rule)
+        public static bool SatisfiesParentTag(string parentTagNameWithoutPrefix, TagMatchingRuleDescriptor rule)
         {
             if (rule == null)
             {
                 throw new ArgumentNullException(nameof(rule));
             }
 
-            if (rule.ParentTag != null && !string.Equals(parentTagName, rule.ParentTag, StringComparison.OrdinalIgnoreCase))
+            if (rule.ParentTag != null && !string.Equals(parentTagNameWithoutPrefix, rule.ParentTag, StringComparison.OrdinalIgnoreCase))
             {
                 return false;
             }

src/Microsoft.CodeAnalysis.Razor.Workspaces/DefaultTagHelperCompletionService.cs

@@ -55,7 +55,13 @@ namespace Microsoft.CodeAnalysis.Razor
             var prefix = documentContext.Prefix ?? string.Empty;
             Debug.Assert(completionContext.CurrentTagName.StartsWith(prefix, StringComparison.OrdinalIgnoreCase));
 
-            var applicableTagHelperBinding = _tagHelperFactsService.GetTagHelperBinding(documentContext, completionContext.CurrentTagName, completionContext.Attributes, completionContext.CurrentParentTagName);
+            var applicableTagHelperBinding = _tagHelperFactsService.GetTagHelperBinding(
+                documentContext,
+                completionContext.CurrentTagName,
+                completionContext.Attributes,
+                completionContext.CurrentParentTagName,
+                parentIsTagHelper: false);
+
             var applicableDescriptors = applicableTagHelperBinding?.Descriptors ?? Enumerable.Empty<TagHelperDescriptor>();
             var unprefixedTagName = completionContext.CurrentTagName.Substring(prefix.Length);
 
@@ -227,11 +233,13 @@ namespace Microsoft.CodeAnalysis.Razor
             }
 
             var prefix = completionContext.DocumentContext.Prefix ?? string.Empty;
+
             var binding = _tagHelperFactsService.GetTagHelperBinding(
                 completionContext.DocumentContext,
                 completionContext.ContainingTagName,
                 completionContext.Attributes,
-                completionContext.ContainingParentTagName);
+                completionContext.ContainingParentTagName,
+                completionContext.ContainingParentIsTagHelper);
 
             if (binding == null)
             {

src/Microsoft.CodeAnalysis.Razor.Workspaces/DefaultTagHelperFactsService.cs

@@ -14,7 +14,8 @@ namespace Microsoft.CodeAnalysis.Razor
             TagHelperDocumentContext documentContext,
             string tagName,
             IEnumerable<KeyValuePair<string, string>> attributes,
-            string parentTag)
+            string parentTag,
+            bool parentIsTagHelper)
         {
             if (documentContext == null)
             {
@@ -39,7 +40,7 @@ namespace Microsoft.CodeAnalysis.Razor
 
             var prefix = documentContext.Prefix;
             var tagHelperBinder = new TagHelperBinder(prefix, descriptors);
-            var binding = tagHelperBinder.GetBinding(tagName, attributes.ToList(), parentTag);
+            var binding = tagHelperBinder.GetBinding(tagName, attributes.ToList(), parentTag, parentIsTagHelper);
 
             return binding;
         }

src/Microsoft.CodeAnalysis.Razor.Workspaces/ElementCompletionContext.cs

@@ -15,6 +15,7 @@ namespace Microsoft.CodeAnalysis.Razor
             string containingTagName,
             IEnumerable<KeyValuePair<string, string>> attributes,
             string containingParentTagName,
+            bool containingParentIsTagHelper,
             Func<string, bool> inHTMLSchema)
         {
             if (documentContext == null)
@@ -37,6 +38,7 @@ namespace Microsoft.CodeAnalysis.Razor
             ContainingTagName = containingTagName;
             Attributes = attributes;
             ContainingParentTagName = containingParentTagName;
+            ContainingParentIsTagHelper = containingParentIsTagHelper;
             InHTMLSchema = inHTMLSchema;
         }
 
@@ -50,6 +52,8 @@ namespace Microsoft.CodeAnalysis.Razor
 
         public string ContainingParentTagName { get; }
 
+        public bool ContainingParentIsTagHelper { get; }
+
         public Func<string, bool> InHTMLSchema { get; }
     }
 }

src/Microsoft.CodeAnalysis.Razor.Workspaces/TagHelperFactsService.cs

@@ -9,7 +9,7 @@ namespace Microsoft.CodeAnalysis.Razor
 {
     public abstract class TagHelperFactsService : ILanguageService
     {
-        public abstract TagHelperBinding GetTagHelperBinding(TagHelperDocumentContext documentContext, string tagName, IEnumerable<KeyValuePair<string, string>> attributes, string parentTag);
+        public abstract TagHelperBinding GetTagHelperBinding(TagHelperDocumentContext documentContext, string tagName, IEnumerable<KeyValuePair<string, string>> attributes, string parentTag, bool parentIsTagHelper);
 
         public abstract IEnumerable<BoundAttributeDescriptor> GetBoundTagHelperAttributes(TagHelperDocumentContext documentContext, string attributeName, TagHelperBinding binding);
 

src/Microsoft.VisualStudio.LanguageServices.Razor/Legacy/DefaultTagHelperCompletionService.cs

@@ -62,7 +62,12 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor
             var prefix = documentContext.Prefix ?? string.Empty;
             Debug.Assert(completionContext.CurrentTagName.StartsWith(prefix, StringComparison.OrdinalIgnoreCase));
 
-            var applicableTagHelperBinding = _tagHelperFactsService.GetTagHelperBinding(documentContext, completionContext.CurrentTagName, completionContext.Attributes, completionContext.CurrentParentTagName);
+            var applicableTagHelperBinding = _tagHelperFactsService.GetTagHelperBinding(
+                documentContext,
+                completionContext.CurrentTagName,
+                completionContext.Attributes,
+                completionContext.CurrentParentTagName);
+
             var applicableDescriptors = applicableTagHelperBinding?.Descriptors ?? Enumerable.Empty<TagHelperDescriptor>();
             var unprefixedTagName = completionContext.CurrentTagName.Substring(prefix.Length);
 

src/Microsoft.VisualStudio.LanguageServices.Razor/Legacy/DefaultTagHelperFactsService.cs

@@ -45,7 +45,7 @@ namespace Microsoft.VisualStudio.LanguageServices.Razor
 
             var prefix = documentContext.Prefix;
             var tagHelperBinder = new TagHelperBinder(prefix, descriptors);
-            var binding = tagHelperBinder.GetBinding(tagName, attributes.ToList(), parentTag);
+            var binding = tagHelperBinder.GetBinding(tagName, attributes.ToList(), parentTag, parentIsTagHelper: false);
 
             return binding;
         }

test/Microsoft.AspNetCore.Razor.Language.Test/Legacy/TagHelperParseTreeRewriterTest.cs

@@ -385,6 +385,99 @@ namespace Microsoft.AspNetCore.Razor.Language.Legacy
                 tagHelperPrefix: "th:");
         }
 
+        [Fact]
+        public void Rewrite_UnderstandsTagHelperPrefixAndAllowedChildrenAndRequireParent()
+        {
+            // Arrange
+            var documentContent = "<th:p><th:strong></th:strong></th:p>";
+            var expectedOutput = new MarkupBlock(
+                new MarkupTagHelperBlock("th:p",
+                    new MarkupTagHelperBlock("th:strong")));
+            var descriptors = new TagHelperDescriptor[]
+            {
+                TagHelperDescriptorBuilder.Create("PTagHelper", "SomeAssembly")
+                    .TagMatchingRuleDescriptor(rule => rule.RequireTagName("p"))
+                    .AllowChildTag("strong")
+                    .Build(),
+                TagHelperDescriptorBuilder.Create("StrongTagHelper", "SomeAssembly")
+                    .TagMatchingRuleDescriptor(rule => rule.RequireTagName("strong").RequireParentTag("p"))
+                    .Build(),
+            };
+
+            // Act & Assert
+            EvaluateData(
+                descriptors,
+                documentContent,
+                expectedOutput,
+                expectedErrors: Enumerable.Empty<RazorError>(),
+                tagHelperPrefix: "th:");
+        }
+
+        [Fact]
+        public void Rewrite_InvalidStructure_UnderstandsTagHelperPrefixAndAllowedChildrenAndRequireParent()
+        {
+            // Arrange
+            var documentContent = "<th:p></th:strong></th:p>";
+            var expectedOutput = new MarkupBlock(
+                new MarkupTagHelperBlock("th:p",
+                    new MarkupTagBlock(
+                        Factory.Markup("</th:strong>"))));
+            var descriptors = new TagHelperDescriptor[]
+            {
+                TagHelperDescriptorBuilder.Create("PTagHelper", "SomeAssembly")
+                    .TagMatchingRuleDescriptor(rule => rule.RequireTagName("p"))
+                    .AllowChildTag("strong")
+                    .Build(),
+                TagHelperDescriptorBuilder.Create("StrongTagHelper", "SomeAssembly")
+                    .TagMatchingRuleDescriptor(rule => rule.RequireTagName("strong").RequireParentTag("p"))
+                    .Build(),
+            };
+            var expectedErrors = new[] {
+                new RazorError(
+                    LegacyResources.FormatTagHelpersParseTreeRewriter_FoundMalformedTagHelper("th:strong"),
+                    absoluteIndex: 8,
+                    lineIndex: 0,
+                    columnIndex: 8,
+                    length: 9),
+            };
+
+            // Act & Assert
+            EvaluateData(
+                descriptors,
+                documentContent,
+                expectedOutput,
+                expectedErrors: expectedErrors,
+                tagHelperPrefix: "th:");
+        }
+
+        [Fact]
+        public void Rewrite_NonTagHelperChild_UnderstandsTagHelperPrefixAndAllowedChildren()
+        {
+            // Arrange
+            var documentContent = "<th:p><strong></strong></th:p>";
+            var expectedOutput = new MarkupBlock(
+                new MarkupTagHelperBlock("th:p",
+                    new MarkupTagBlock(
+                        Factory.Markup("<strong>")),
+                    new MarkupTagBlock(
+                        Factory.Markup("</strong>"))));
+            var descriptors = new TagHelperDescriptor[]
+            {
+                TagHelperDescriptorBuilder.Create("PTagHelper", "SomeAssembly")
+                    .TagMatchingRuleDescriptor(rule => rule.RequireTagName("p"))
+                    .AllowChildTag("strong")
+                    .Build(),
+            };
+
+            // Act & Assert
+            EvaluateData(
+                descriptors,
+                documentContent,
+                expectedOutput,
+                expectedErrors: Enumerable.Empty<RazorError>(),
+                tagHelperPrefix: "th:");
+        }
+
         public static TheoryData InvalidHtmlScriptBlockData
         {
             get

test/Microsoft.AspNetCore.Razor.Language.Test/TagHelperBinderTest.cs

@@ -28,7 +28,8 @@ namespace Microsoft.AspNetCore.Razor.Language
             var bindingResult = tagHelperBinder.GetBinding(
                 tagName: "th:div",
                 attributes: expectedAttributes,
-                parentTagName: "body");
+                parentTagName: "body",
+                parentIsTagHelper: false);
 
             // Assert
             Assert.Equal(expectedDescriptors, bindingResult.Descriptors, TagHelperDescriptorComparer.CaseSensitive);
@@ -109,7 +110,8 @@ namespace Microsoft.AspNetCore.Razor.Language
             var bindingResult = tagHelperBinder.GetBinding(
                 tagName,
                 attributes: Array.Empty<KeyValuePair<string, string>>(),
-                parentTagName: parentTagName);
+                parentTagName: parentTagName,
+                parentIsTagHelper: false);
 
             // Assert
             Assert.Equal((IEnumerable<TagHelperDescriptor>)expectedDescriptors, bindingResult.Descriptors, TagHelperDescriptorComparer.CaseSensitive);
@@ -272,7 +274,7 @@ namespace Microsoft.AspNetCore.Razor.Language
             var tagHelperBinder = new TagHelperBinder(null, (IReadOnlyList<TagHelperDescriptor>)availableDescriptors);
 
             // Act
-            var bindingResult = tagHelperBinder.GetBinding(tagName, providedAttributes, parentTagName: "p");
+            var bindingResult = tagHelperBinder.GetBinding(tagName, providedAttributes, parentTagName: "p", parentIsTagHelper: false);
 
             // Assert
             Assert.Equal((IEnumerable<TagHelperDescriptor>)expectedDescriptors, bindingResult?.Descriptors, TagHelperDescriptorComparer.CaseSensitive);
@@ -292,7 +294,8 @@ namespace Microsoft.AspNetCore.Razor.Language
             var bindingResult = tagHelperBinder.GetBinding(
                 tagName: "th",
                 attributes: Array.Empty<KeyValuePair<string, string>>(),
-                parentTagName: "p");
+                parentTagName: "p",
+                parentIsTagHelper: false);
 
             // Assert
             Assert.Null(bindingResult);
@@ -312,11 +315,13 @@ namespace Microsoft.AspNetCore.Razor.Language
             var bindingResultDiv = tagHelperBinder.GetBinding(
                 tagName: "th:div",
                 attributes: Array.Empty<KeyValuePair<string, string>>(),
-                parentTagName: "p");
+                parentTagName: "p",
+                parentIsTagHelper: false);
             var bindingResultSpan = tagHelperBinder.GetBinding(
                 tagName: "th:span",
                 attributes: Array.Empty<KeyValuePair<string, string>>(),
-                parentTagName: "p");
+                parentTagName: "p",
+                parentIsTagHelper: false);
 
             // Assert
             var descriptor = Assert.Single(bindingResultDiv.Descriptors);
@@ -339,7 +344,8 @@ namespace Microsoft.AspNetCore.Razor.Language
             var bindingResult = tagHelperBinder.GetBinding(
                 tagName: "th:div",
                 attributes: Array.Empty<KeyValuePair<string, string>>(),
-                parentTagName: "p");
+                parentTagName: "p",
+                parentIsTagHelper: false);
 
             // Assert
             var descriptor = Assert.Single(bindingResult.Descriptors);
@@ -362,7 +368,8 @@ namespace Microsoft.AspNetCore.Razor.Language
             var bindingResult = tagHelperBinder.GetBinding(
                 tagName: "div",
                 attributes: Array.Empty<KeyValuePair<string, string>>(),
-                parentTagName: "p");
+                parentTagName: "p",
+                parentIsTagHelper: false);
 
             // Assert
             Assert.Null(bindingResult);
@@ -385,7 +392,8 @@ namespace Microsoft.AspNetCore.Razor.Language
             var tagHelperBinding = tagHelperBinder.GetBinding(
                 tagName: "foo",
                 attributes: Array.Empty<KeyValuePair<string, string>>(),
-                parentTagName: "p");
+                parentTagName: "p",
+                parentIsTagHelper: false);
 
             // Assert
             Assert.Null(tagHelperBinding);
@@ -411,11 +419,13 @@ namespace Microsoft.AspNetCore.Razor.Language
             var divBinding = tagHelperBinder.GetBinding(
                 tagName: "div",
                 attributes: Array.Empty<KeyValuePair<string, string>>(),
-                parentTagName: "p");
+                parentTagName: "p",
+                parentIsTagHelper: false);
             var spanBinding = tagHelperBinder.GetBinding(
                 tagName: "span",
                 attributes: Array.Empty<KeyValuePair<string, string>>(),
-                parentTagName: "p");
+                parentTagName: "p",
+                parentIsTagHelper: false);
 
             // Assert
             // For divs
@@ -443,7 +453,8 @@ namespace Microsoft.AspNetCore.Razor.Language
             var bindingResult = tagHelperBinder.GetBinding(
                 tagName: "div",
                 attributes: Array.Empty<KeyValuePair<string, string>>(),
-                parentTagName: "p");
+                parentTagName: "p",
+                parentIsTagHelper: false);
 
             // Assert
             var descriptor = Assert.Single(bindingResult.Descriptors);
@@ -470,7 +481,8 @@ namespace Microsoft.AspNetCore.Razor.Language
             var binding = tagHelperBinder.GetBinding(
                 tagName: "div",
                 attributes: Array.Empty<KeyValuePair<string, string>>(),
-                parentTagName: "p");
+                parentTagName: "p",
+                parentIsTagHelper: false);
 
             // Assert
             var boundDescriptor = Assert.Single(binding.Descriptors);
@@ -479,5 +491,34 @@ namespace Microsoft.AspNetCore.Razor.Language
             var boundRule = Assert.Single(boundRules);
             Assert.Equal("div", boundRule.TagName);
         }
+
+        [Fact]
+        public void GetBinding_PrefixedParent_ReturnsBinding()
+        {
+            // Arrange
+            var divDescriptor = TagHelperDescriptorBuilder.Create("foo1", "SomeAssembly")
+                .TagMatchingRuleDescriptor(rule => rule.RequireTagName("div").RequireParentTag("p"))
+                .Build();
+            var pDescriptor = TagHelperDescriptorBuilder.Create("foo2", "SomeAssembly")
+                .TagMatchingRuleDescriptor(rule => rule.RequireTagName("p"))
+                .Build();
+            var descriptors = new[] { divDescriptor, pDescriptor };
+            var tagHelperBinder = new TagHelperBinder("th:", descriptors);
+
+            // Act
+            var bindingResult = tagHelperBinder.GetBinding(
+                tagName: "th:div",
+                attributes: Array.Empty<KeyValuePair<string, string>>(),
+                parentTagName: "th:p",
+                parentIsTagHelper: true);
+
+            // Assert
+            var boundDescriptor = Assert.Single(bindingResult.Descriptors);
+            Assert.Same(divDescriptor, boundDescriptor);
+            var boundRules = bindingResult.GetBoundRules(boundDescriptor);
+            var boundRule = Assert.Single(boundRules);
+            Assert.Equal("div", boundRule.TagName);
+            Assert.Equal("p", boundRule.ParentTag);
+        }
     }
 }

test/Microsoft.CodeAnalysis.Razor.Workspaces.Test/DefaultTagHelperCompletionServiceTest.cs

@@ -927,6 +927,7 @@ namespace Microsoft.CodeAnalysis.Razor
             IEnumerable<string> existingCompletions,
             string containingTagName,
             string containingParentTagName = "body",
+            bool containingParentIsTagHelper = false,
             string tagHelperPrefix = "")
         {
             var documentContext = TagHelperDocumentContext.Create(tagHelperPrefix, descriptors);
@@ -936,6 +937,7 @@ namespace Microsoft.CodeAnalysis.Razor
                 containingTagName,
                 attributes: Enumerable.Empty<KeyValuePair<string, string>>(),
                 containingParentTagName: containingParentTagName,
+                containingParentIsTagHelper: containingParentIsTagHelper,
                 inHTMLSchema: (tag) => tag == "strong" || tag == "b" || tag == "bold" || tag == "li" || tag == "div");
 
             return completionContext;

test/Microsoft.CodeAnalysis.Razor.Workspaces.Test/DefaultTagHelperFactsServiceTest.cs

@@ -27,7 +27,7 @@ namespace Microsoft.CodeAnalysis.Razor
             var service = new DefaultTagHelperFactsService();
 
             // Act
-            var binding = service.GetTagHelperBinding(documentContext, "!a", Enumerable.Empty<KeyValuePair<string, string>>(), parentTag: null);
+            var binding = service.GetTagHelperBinding(documentContext, "!a", Enumerable.Empty<KeyValuePair<string, string>>(), parentTag: null, parentIsTagHelper: false);
 
             // Assert
             Assert.Null(binding);
@@ -73,7 +73,7 @@ namespace Microsoft.CodeAnalysis.Razor
             };
 
             // Act
-            var binding = service.GetTagHelperBinding(documentContext, "a", attributes, parentTag: "p");
+            var binding = service.GetTagHelperBinding(documentContext, "a", attributes, parentTag: "p", parentIsTagHelper: false);
 
             // Assert
             var descriptor = Assert.Single(binding.Descriptors);
@@ -109,7 +109,7 @@ namespace Microsoft.CodeAnalysis.Razor
             };
             var documentContext = TagHelperDocumentContext.Create(string.Empty, documentDescriptors);
             var service = new DefaultTagHelperFactsService();
-            var binding = service.GetTagHelperBinding(documentContext, "a", Enumerable.Empty<KeyValuePair<string, string>>(), parentTag: null);
+            var binding = service.GetTagHelperBinding(documentContext, "a", Enumerable.Empty<KeyValuePair<string, string>>(), parentTag: null, parentIsTagHelper: false);
 
             // Act
             var descriptors = service.GetBoundTagHelperAttributes(documentContext, "asp-route-something", binding);
@@ -144,7 +144,7 @@ namespace Microsoft.CodeAnalysis.Razor
             };
             var documentContext = TagHelperDocumentContext.Create(string.Empty, documentDescriptors);
             var service = new DefaultTagHelperFactsService();
-            var binding = service.GetTagHelperBinding(documentContext, "input", Enumerable.Empty<KeyValuePair<string, string>>(), parentTag: null);
+            var binding = service.GetTagHelperBinding(documentContext, "input", Enumerable.Empty<KeyValuePair<string, string>>(), parentTag: null, parentIsTagHelper: false);
 
             // Act
             var descriptors = service.GetBoundTagHelperAttributes(documentContext, "asp-for", binding);