Roslyn does not provide a way to suppress nullability warnings on the left side of a compound assignments

[TessenR]

Version Used:

Branch master (19 Jun 2019)
Latest commit e1146e by Sam Harwell:
Merge pull request #36366 from sharwell/order-naming-rules

Implement subset-based ordering for naming rules

Steps to Reproduce:

#nullable enable
class D
{
    void M(D? d)
    {
        d += d; // CS8604
        // d! += d; // does not compile
        // (d)! += d; // does not compile
    }
    
    public static extern D operator +(D d1, D? d2);
}

Expected Behavior:
You can suppress nullability warnings in the compound assignment since nullability analysis might be incorrect.

Actual Behavior:
There's no way to suppress it with a !.

[RikkiGibson]

@jcouv I believe this relates to #27522 and #33869