Remove _isBlocked constraint from StreamReader.Peek()

src/libraries/Common/tests/System/IO/ReadOneAtATimeStream.cs

@@ -0,0 +1,45 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+namespace System.IO.Tests
+{
+    public class ReadOneAtATimeStream : Stream
+    {
+        MemoryStream _memoryStream;
+
+        public ReadOneAtATimeStream(byte[] buffer)
+        {
+            _memoryStream = new MemoryStream(buffer);
+        }
+
+        public override bool CanRead => true;
+
+        public override bool CanSeek => false;
+
+        public override bool CanWrite => false;
+
+        public override long Length => _memoryStream.Length;
+
+        public override long Position
+        {
+            get => _memoryStream.Position;
+            set => throw new NotSupportedException();
+        }
+
+        public override int Read(byte[] buffer, int offset, int count)
+        {
+            if (count == 0 || Position == Length)
+            {
+                return 0;
+            }
+
+            _memoryStream.ReadExactly(buffer, offset, 1);
+            return 1;
+        }
+
+        public override void Flush() { }
+        public override long Seek(long offset, SeekOrigin origin) => throw new NotSupportedException();
+        public override void SetLength(long value) => throw new NotSupportedException();
+        public override void Write(byte[] buffer, int offset, int count) => throw new NotSupportedException();
+    }
+}

src/libraries/System.IO/tests/StreamReader/StreamReaderTests.cs

@@ -174,6 +174,30 @@ public void TestPeek()
             }
         }
 
+        [Fact]
+        public void TestPeekReadOneByteAtATime()
+        {
+            byte[] testData = new byte[] { 72, 69, 76, 76, 79 };
+
+            using var stream = new ReadOneAtATimeStream(testData);
+            using var sr = new StreamReader(stream);
+
+            for (int i = 0; i < testData.Length; i++)
+            {
+                Assert.Equal(i, stream.Position);
+
+                int tmp = sr.Peek();
+                Assert.Equal(testData[i], tmp);
+
+                tmp = sr.Read(); 
+                Assert.Equal(testData[i], tmp);
+            }
+
+            Assert.Equal(stream.Position, stream.Length);
+            Assert.Equal(-1, sr.Peek());
+            Assert.Equal(-1, sr.Read());
+        }
+
         [Fact]
         public void ArgumentNullOnNullArray()
         {

src/libraries/System.IO/tests/System.IO.Tests.csproj

@@ -53,6 +53,7 @@
     <Compile Include="$(CommonTestPath)System\IO\CallTrackingStream.cs" Link="Common\System\IO\CallTrackingStream.cs" />
     <Compile Include="$(CommonTestPath)System\IO\DelegateStream.cs" Link="Common\System\IO\DelegateStream.cs" />
     <Compile Include="$(CommonTestPath)System\IO\WrappedMemoryStream.cs" Link="Common\System\IO\WrappedMemoryStream.cs" />
+    <Compile Include="$(CommonTestPath)System\IO\ReadOneAtATimeStream.cs" Link="Common\System\IO\ReadOneAtATimeStream.cs" />
     <Compile Include="TestDataProvider\TestDataProvider.cs" />
     <Compile Include="TextReader\CharArrayTextReader.cs" />
     <Compile Include="TextReader\TextReaderTests.cs" />

src/libraries/System.Private.CoreLib/src/System/IO/StreamReader.cs

@@ -320,7 +320,7 @@ public override int Peek()
 
             if (_charPos == _charLen)
             {
-                if (_isBlocked || ReadBuffer() == 0)
+                if (ReadBuffer() == 0)
                 {
                     return -1;
                 }