Nullable Reference Types: Compiler should not report CS8603 when assignment exists in the right-hand side of a null coalescing operator

[PathogenDavid]

Version Used: 05/14/18 Nullable Reference Types Preview (csc reports 2.8.0.62830 (e595ee27)) with Visual Studio 15.7.1

Demonstration Code:

I've created PathogenPlayground/NullCoalescingOperatorCs8603 to demonstrate the issue.

Steps to Reproduce:

Compile the following code, where Foo is a class:

private static Foo? _foo = null;
public static Foo FooProperty
{
    get => _foo ?? (_foo = new Foo()); // warning CS8603: Possible null reference return.
}

public static Foo FooProperty2
{
    get
    {
        Foo? ret = null;
        return ret ?? (ret = new Foo()); // warning CS8603: Possible null reference return.
    }
}

Edit: The issue also happens when the null coalescing operator is not in play:

private static Foo? _foo4 = null;
public static Foo FooProperty4
{
    get
    {
        if (_foo4 == null)
        { _foo4 = new Foo(); }

        return _foo4; // warning CS8603: Possible null reference return.
    }
}

Expected Behavior:

No warnings are reported.

Actual Behavior:

Possible null reference return is reported for the return statement in each property.

---

The readme on PathogenPlayground/NullCoalescingOperatorCs8603 contains some additional details.

FooProperty2 exists to make sure the problem isn't somehow related to #9978

[PathogenDavid]

I've also updated PathogenPlayground/NullCoalescingOperatorCs860 with non-static variants of the properties since I found out that the behavior differs for FooProperty4 when the property and field aren't static.

@cston I suspect that the FooProperty4 issue is more related to #23063 than the null-coalescing problems. Would you prefer I submit a separate issue for it?

[jcouv]

Confirmed.
I think this is indeed a bug. Thanks for reporting.

It seems caused by the field being static (issue tracked by #26651)

[jaredpar]

Issue appears to be fixed now. Assigning to @jaredpar to add a regression test.