Fix another Sequence<T> AdvanceTo bug

Author: AArnott

src/Nerdbank.Streams.Tests/SequenceTests.cs

@@ -284,6 +284,26 @@ public void AdvanceTo_InterweavedWith_Advance()
         Assert.Equal(origLastChar, seq.AsReadOnlySequence.Slice(origLastCharPosition, 1).First.Span[0]);
     }
 
+    [Fact]
+    public void AdvanceTo_InterweavedWith_Advance2()
+    {
+        // use the mock pool so that we can predict the actual array size will not exceed what we ask for.
+        var seq = new Sequence<int>(new MockPool<int>());
+
+        seq.GetSpan(10);
+        seq.Advance(10);
+
+        seq.AdvanceTo(seq.AsReadOnlySequence.GetPosition(3));
+
+        seq.GetSpan(10);
+        seq.Advance(10);
+
+        seq.GetSpan(10);
+        seq.Advance(10);
+
+        Assert.Equal(10 - 3 + 10 + 10, seq.AsReadOnlySequence.Length);
+    }
+
     [Fact]
     public void Dispose_ReturnsArraysToPool()
     {

src/Nerdbank.Streams/Sequence`1.cs

@@ -346,7 +346,7 @@ internal void SetNext(SequenceSegment segment)
                 Requires.NotNull(segment, nameof(segment));
 
                 this.Next = segment;
-                segment.RunningIndex = this.RunningIndex + this.Length;
+                segment.RunningIndex = this.RunningIndex + this.End;
             }
 
             internal void AdvanceTo(int offset)