CA1859 erroneously recommends nullable return type

[stephentoub]

Analyzer

Diagnostic ID: CA1859

Analyzer source

6ae2683

Describe the bug

The analyzer recommends changing a non-nullable return type to a nullable one.

Steps To Reproduce

[Fact]
public static async Task Repro()
{
    const string Source = @"
        #nullable enable

        using System;
        using System.IO;

        namespace Example
        {
            internal class C
            {
                private MemoryStream? _stream;

                private Stream GetStream()
                {
                    return _stream ?? Create();
                }

                private MemoryStream Create() => new MemoryStream();
            }
        }
        ";

    await TestCSAsync(Source);
}

Expected behavior

info CA1859: Change return type of method 'GetStream' from 'System.IO.Stream' to 'System.IO.MemoryStream' for improved performance

Actual behavior

info CA1859: Change return type of method 'GetStream' from 'System.IO.Stream' to 'System.IO.MemoryStream?' for improved performance

cc: @geeknoid, @buyaa-n

[Youssef1313]

Closing as fixed per #6418 (comment)