Fix index of pattern match.

src/Features/CSharp/Portable/Completion/CompletionProviders/AttributeNamedParameterCompletionProvider.cs

 // Copyright (c) Microsoft.  All Rights Reserved.  Licensed under the Apache License, Version 2.0.  See License.txt in the project root for license information.
 
+using System;
 using System.Collections.Generic;
 using System.Collections.Immutable;
 using System.Linq;
@@ -14,7 +15,6 @@
 using Microsoft.CodeAnalysis.Shared.Extensions;
 using Microsoft.CodeAnalysis.Text;
 using Roslyn.Utilities;
-using System;
 
 namespace Microsoft.CodeAnalysis.CSharp.Completion.Providers
 {

src/Features/Core/Portable/Completion/CompletionHelper.cs

@@ -78,11 +78,13 @@ public bool MatchesPattern(string text, string pattern, CultureInfo culture)
             var lastDotIndex = completionItemText.LastIndexOf('.');
             if (lastDotIndex >= 0)
             {
-                var textAfterLastDot = completionItemText.Substring(lastDotIndex + 1);
+                var afterDotPosition = lastDotIndex + 1;
+                var textAfterLastDot = completionItemText.Substring(afterDotPosition);
+
                 var match = GetMatchWorker(textAfterLastDot, pattern, includeMatchSpans, culture);
                 if (match != null)
                 {
-                    return match;
+                    return AdjustMatchedSpans(match.Value, afterDotPosition);
                 }
             }
 
@@ -91,6 +93,11 @@ public bool MatchesPattern(string text, string pattern, CultureInfo culture)
             return GetMatchWorker(completionItemText, pattern, includeMatchSpans, culture);
         }
 
+        private PatternMatch? AdjustMatchedSpans(PatternMatch value, int offset)
+            => value.MatchedSpans.IsDefaultOrEmpty
+                ? value
+                : value.WithMatchedSpans(value.MatchedSpans.SelectAsArray(s => new TextSpan(s.Start + offset, s.Length)));
+
         private PatternMatch? GetMatchWorker(
             string completionItemText, string pattern,
             bool includeMatchSpans, CultureInfo culture)

src/Workspaces/Core/Portable/PatternMatching/PatternMatch.cs

@@ -64,10 +64,11 @@ internal struct PatternMatch : IComparable<PatternMatch>
             }
         }
 
+        public PatternMatch WithMatchedSpans(ImmutableArray<TextSpan> matchedSpans)
+            => new PatternMatch(Kind, _punctuationStripped, IsCaseSensitive, matchedSpans, CamelCaseWeight);
+
         public int CompareTo(PatternMatch other)
-        {
-            return CompareTo(other, ignoreCase: false);
-        }
+            => CompareTo(other, ignoreCase: false);
 
         public int CompareTo(PatternMatch other, bool ignoreCase)
         {
@@ -109,9 +110,7 @@ private static int CompareCase(PatternMatch result1, PatternMatch result2, bool
         }
 
         private static int CompareType(PatternMatch result1, PatternMatch result2)
-        {
-            return result1.Kind - result2.Kind;
-        }
+            => result1.Kind - result2.Kind;
 
         private static int CompareCamelCase(PatternMatch result1, PatternMatch result2)
         {