abstract class Foo
{
    public Foo() { /* .. */ }
}

May become:

abstract class Foo
{
    protected Foo() { /* .. */ }
}

or

abstract class Foo
{
    protected internal Foo() { /* .. */ }
}

or

abstract class Foo
{
    private Foo() { /* .. */ }
}

It may only become private if no deriving class is already referencing it. And protected internal if no deriving class outside the assembly. In other words, should not break the code.
Category: Usage
Severity: Warning
Diagnostic Id: CC0060