CA1859 improperly handles explicit interface implementations on a parent class

[jnyrup]

Analyzer

Diagnostic ID: CA1859: Use concrete types when possible for improved performance

Analyzer source

SDK: Built-in CA analyzers in .NET 8 Preview 5 SDK

Version: 8.0.100-preview.5.23303.2

Describe the bug

#6416 described a false positive when a type explicitly implemented the interface method being invoked.
This issue is about when a parent class explicitly implements the interface method.

Steps To Reproduce

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net7.0</TargetFramework>
    <AnalysisLevel>latest</AnalysisLevel>
  </PropertyGroup>
</Project>

class Class1
{
    public void NoWarning()
    {
        I obj = new Base();
        obj.M();
    }

    public void FalseWarning()
    {
        I obj = new Derived(); // <-- triggers CA1859
        obj.M();
    }
}

interface I
{
    void M();
}

class Base : I
{
    void I.M()
    {
    }
}

class Derived : Base
{
}

Expected behavior

CA1859 should not be triggered as M() is only explicitly implemented.

Actual behavior

CA1859 is triggered.

CA1859	Change type of variable 'obj' from 'I' to 'Derived' for improved performance

Additional context