"Implement abstract class" should add "where T : default" when a parameter is covariant in T?

[SolalPirelli]

Version Used: Visual Studio 17.2.5 (VisualStudio.17.Release/17.2.5+32616.157 ; C# Tools 4.2.0-4.22281.5+8d3180e5f00d42f0f0295165f756f368f0cbfa44)

Steps to Reproduce:

1. Write the following code:

interface I<out T> { }

abstract class C
{
    protected abstract void M1<T>(T? t);
    protected abstract void M2<T>(I<T?> i);
}

class C2 : C
{
}

2. Use "Implement abstract class" on C2

Expected Behavior:

The resulting code should compile.

Actual Behavior:

M1 is correctly implemented but M2 is not, as it is missing where T : default and thus does not compile:

    protected override void M1<T>(T? t) where T : default
    {
        throw new NotImplementedException();
    }

    protected override void M2<T>(I<T?> i) // <- this does not compile
    {
        throw new NotImplementedException();
    }

---

A more realistic example for is using a Func<T?> as parameter.

It took me a while to even understand what the issue was, since the compiler doesn't say anything about where T : default being something you can do in the resulting error messages. Perhaps the compiler could detect that there is an override that would work if it had where T : default and warn about that?

[Youssef1313]

I think I missed this case in #53318.

[SolalPirelli]

This happens for implementing interfaces implicitly as well:

interface I<out T> { }

interface I2
{
    void M1<T>(I<T?> i);
}

class C : I2
{
}

"Implement interface" produces valid code, "Implement all members explicitly" does not. (I like how the preview for the latter already shows the red squiggles within the preview -- how cool is that? 😄 )