CA1859 erroneously suggests changing return type to one of multiple possible return types

[stephentoub]

Analyzer

Diagnostic ID: CA1859

Analyzer source

6ae2683

Describe the bug

A method can return both the base abstract type and a derived type, and the analyzer erroneously suggests changing the return type to the derived type.

Steps To Reproduce

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

        using System;
        using System.IO;

        namespace Example
        {
            internal static class C
            {
                private static Stream GetStream(int i)
                {
                    if (i == 0)
                    {
                        return Stream.Null;
                    }

                    return new MyStream();
                }
            }
        }

        public class MyStream : MemoryStream { }
        ";

    await TestCSAsync(Source);
}

Expected behavior

No warning.

Actual behavior

    // /0/Test0.cs(11,47): info CA1859: Change return type of method 'GetStream' from 'System.IO.Stream' to 'MyStream' for improved performance
VerifyCS.Diagnostic(UseConcreteTypeAnalyzer.CA1859).WithSpan(11, 47, 11, 56).WithArguments("GetStream", "System.IO.Stream", "MyStream"),

cc: @geeknoid, @buyaa-n

[Youssef1313]

Closing as fixed per #6418 (comment)