ApiCompat does not show all breaking changes for --respect-internals option

I have a scenario where the contract has an abstract class with an internal ctor. For example:

public abstract class Utility
{
    internal Utility(){ } //internal ctor
    public abstract Add(int a, int b);
}

In the implementation, I have marked the ctor as protected and also added a new abstract method.

public abstract class Utility
{
    protected Utility(){ } //protected ctor
    public abstract Add(int a, int b);
    public abstract Subtract(int a, int b);
}

When I run ApiCompat, I provide the --respect-internals option.

Expectation: ApiCompat fails with the error CannotAddAbstractMembers as I have added a new abstract method

Actual Behavior: ApiCompat only reports this error CannotMakeMoreVisible for the class

[ericstj]

As @Anipik mentions it’s treating internal constructors as “effectively sealed” https://github.com/dotnet/arcade/blob/cde36344adbc1c9808b1c3e7419f0ce3c3e5cd04/src/Microsoft.Cci.Extensions/Extensions/CSharp/CSharpCciExtensions.cs#L669-L686

So perhaps that needs to change to catch this specific case. It’s not effectively sealed if we’re considering internal API as part of the compatibility contract.

[Anipik]

we pass --respect-internals as apicompat option to catch breaking changes in internaltypes as well https://github.com/dotnet/arcade/blob/main/src/Microsoft.DotNet.ApiCompat/src/ApiCompatRunner.cs#L80