Brace matching has code to check for matching spans left of the caret - to...

Brace matching has code to check for matching spans left of the caret - to deal with cases like {$$()}. We need to protect against negative numbers when caret is at position 0. (changeset 1405386)

src/EditorFeatures/Core/Implementation/BraceMatching/BraceMatchingService.cs

@@ -26,6 +26,12 @@ internal class BraceMatchingService : IBraceMatchingService
 
         public async Task<BraceMatchingResult?> GetMatchingBracesAsync(Document document, int position, CancellationToken cancellationToken)
         {
+            var text = await document.GetTextAsync(cancellationToken).ConfigureAwait(false);
+            if (position < 0 || position > text.Length)
+            {
+                throw new ArgumentException(nameof(position));
+            }
+
             var matchers = _braceMatchers.Where(b => b.Metadata.Language == document.Project.Language);
             foreach (var matcher in matchers)
             {

src/EditorFeatures/Core/Shared/Extensions/IBraceMatchingServiceExtensions.cs

@@ -18,7 +18,13 @@ internal static class IBraceMatchingServiceExtensions
             var braces1 = await service.GetMatchingBracesAsync(document, position, cancellationToken).ConfigureAwait(false);
 
             // These are the matching spans when checking the token to the left of the position.
-            var braces2 = await service.GetMatchingBracesAsync(document, position - 1, cancellationToken).ConfigureAwait(false);
+            BraceMatchingResult? braces2 = null;
+
+            // Ensure caret is valid at left of position.
+            if (position > 0)
+            {
+                braces2 = await service.GetMatchingBracesAsync(document, position - 1, cancellationToken).ConfigureAwait(false);
+            }
 
             // Favor matches where the position is on the outside boundary of the braces. i.e. if we
             // have:  {^()}