"Implement abstract class" should add the right generic constraint when a parameter is covariant in T? and constrained to be a subtype

[SolalPirelli]

Similar to #62092 but even more specific. To be honest I don't even understand why the actual and expected behaviors here fail and succeed to compile respectively.

Version Used:

• Microsoft Visual Studio Community 2022 Version 17.8.3 VisualStudio.17.Release/17.8.3+34330.188
• C# Tools 4.8.0-7.23572.1+7b75981cf3bd520b86ec4ed00ec156c8bc48e4eb

Steps to Reproduce:

Given the following code, in a project with nullable enabled:

interface I<out T> { }

class C { }

abstract class Problem
{
    protected abstract void M<T>(I<T?> i) where T : C;
}

class Bad : Problem
{
}

Apply "Implement abstract class" on Bad.

Expected Behavior:
The following code compiles, so I think that's what should be generated:

    protected override void M<T>(I<T?> i) where T : class
    {
        throw new System.NotImplementedException();
    }

Actual Behavior:
The following is generated, which does not compile due to the generic constraint:

    protected override void M<T>(I<T?> i) where T : default
    {
        throw new System.NotImplementedException();
    }