Fix mhutch#26

Ensure the diagnostic on unexpected < after an opening tag is covering the second <.

Core/Parser/XmlRootState.cs

@@ -89,8 +89,8 @@ public override XmlParserState PushChar (char c, XmlParserContext context, ref s
 					context.Diagnostics?.LogError (
 						"Incomplete tag opening; encountered unexpected '<'.",
 						TextSpan.FromBounds (
-							context.Position - LengthFromOpenBracket (context) - 1,
-							context.Position - 1
+							context.Position + 1 - LengthFromOpenBracket (context),
+							context.Position + 1
 						)
 					);
 				context.StateTag = BRACKET;

Tests/Parser/ParsingTests.cs

@@ -462,5 +462,16 @@ public void InvalidNameState ()
 			parser.Parse (docTxt);
 			parser.AssertErrorCount (2);
 		}
-  }
+
+		[Test]
+		public void TwoOpenAngles ()
+		{
+			var docTxt = "<<";
+			var parser = new XmlTreeParser (CreateRootState ());
+			parser.Parse (docTxt);
+			var diagnostic = parser.GetContext ().Diagnostics.Single ();
+			Assert.AreEqual (1, diagnostic.Span.Start);
+			Assert.AreEqual (1, diagnostic.Span.Length);
+		}
+	}
 }

Tests/Parser/TestXmlParser.cs

@@ -89,6 +89,12 @@ public static void Parse (this XmlParser parser, string doc, char trigger = '$',
 				}
 			}
 			Assert.AreEqual (asserts.Length, assertNo);
+
+			var diagnostics = context.Diagnostics;
+			foreach (var diagnostic in diagnostics) {
+				Assert.GreaterOrEqual (diagnostic.Span.Start, 0);
+				Assert.GreaterOrEqual (diagnostic.Span.Length, 0);
+			}
 		}
 
 		public static XObject PeekSpine (this XmlParser parser) => parser.GetContext ().Nodes.Peek ();