Enable `TagHelper`s with `[RestrictChildren]` to log errors for no tag name tags.

- Added test cases for situations when tags have no names.

#534

src/Microsoft.AspNet.Razor/Parser/TagHelpers/TagHelperParseTreeRewriter.cs

@@ -308,7 +308,18 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
         {
             if (_currentTagHelperTracker?.AllowedChildren != null)
             {
-                OnAllowedChildrenTagError(_currentTagHelperTracker, tagBlock, errorSink);
+                var tagName = GetTagName(tagBlock);
+
+                // Treat partial tags such as '</' which have no tag names as content based errors.
+                if (string.IsNullOrEmpty(tagName))
+                {
+                    Debug.Assert(tagBlock.Children.First() is Span);
+
+                    ValidateParentTagHelperAllowsContent(tagBlock.Children.First() as Span, errorSink);
+                    return;
+                }
+
+                OnAllowedChildrenTagError(_currentTagHelperTracker, tagName, tagBlock, errorSink);
             }
         }
 
@@ -319,16 +330,16 @@ namespace Microsoft.AspNet.Razor.Parser.TagHelpers.Internal
             if (currentlyAllowedChildren != null &&
                 !currentlyAllowedChildren.Contains(tagName, StringComparer.OrdinalIgnoreCase))
             {
-                OnAllowedChildrenTagError(_currentTagHelperTracker, tagBlock, errorSink);
+                OnAllowedChildrenTagError(_currentTagHelperTracker, tagName, tagBlock, errorSink);
             }
         }
 
         private static void OnAllowedChildrenTagError(
             TagHelperBlockTracker tracker,
+            string tagName,
             Block tagBlock,
             ErrorSink errorSink)
         {
-            var tagName = GetTagName(tagBlock);
             var allowedChildrenString = string.Join(", ", tracker.AllowedChildren);
             var errorMessage = RazorResources.FormatTagHelperParseTreeRewriter_InvalidNestedTag(
                 tagName,

test/Microsoft.AspNet.Razor.Test/TagHelpers/TagHelperParseTreeRewriterTest.cs

@@ -19,11 +19,9 @@ namespace Microsoft.AspNet.Razor.Test.TagHelpers
     public class TagHelperParseTreeRewriterTest : TagHelperRewritingTestBase
     {
         [Fact]
-        public void Rewrite_UnderstandsTagHelperPrefixAndRestrictedChildren()
+        public void Rewrite_UnderstandsTagHelperPrefixAndAllowedChildren()
         {
             // Arrange
-            var factory = CreateDefaultSpanFactory();
-            var blockFactory = new BlockFactory(factory);
             var documentContent = "<th:p><th:strong></th:strong></th:p>";
             var expectedOutput = new MarkupBlock(
                 new MarkupTagHelperBlock("th:p",
@@ -683,6 +681,28 @@ namespace Microsoft.AspNet.Razor.Test.TagHelpers
                             nestedTagError("br", "p", "custom", 51, 2),
                             nestedContentError("p", "custom", 56, 9)
                         }
+                    },
+                    {
+                        "<p></</p>",
+                        new[] { "custom" },
+                        new MarkupBlock(
+                            new MarkupTagHelperBlock("p",
+                                blockFactory.MarkupTagBlock("</"))),
+                        new[]
+                        {
+                            nestedContentError("p", "custom", 3, 2),
+                        }
+                    },
+                    {
+                        "<p><</p>",
+                        new[] { "custom" },
+                        new MarkupBlock(
+                            new MarkupTagHelperBlock("p",
+                                blockFactory.MarkupTagBlock("<"))),
+                        new[]
+                        {
+                            nestedContentError("p", "custom", 3, 1),
+                        }
                     }
                 };
             }