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Looks like this is ready to review. I half expected worms to come out of this can but the CI looks reasonable.

Will this also affect the behaviour of cctors without beforefieldinit?

new Foo().X = 1; // this should print "1" as the cctor of Foo is not beforefieldinit

class Foo
{
    static Foo() { } // opt-out beforefieldinit
    public static Bar bar = new();
    
    public int X;
}

class Bar
{
    public Bar()
    {
        Console.WriteLine(1);
    }
}

I believe some applications rely on this behavior to do initialization.

The instance constructor is responsible for triggering the cctor in your example. The instance field access (on non-null this pointer) assumes that the cctor was triggered when the instance was created.