VSTHRD110 does not trigger when using null-conditional member access

[AlexVallat]

Bug description

If the async method is called using null-conditional member access, no warning (neither VSTHRD110 nor CS4014) is raised.

Repro steps

class Example
{
    void Foo(Example mayNotBeNull, Example? mayBeNull)
    {
        DoStuffAsync();
        mayNotBeNull.DoStuffAsync();
        mayBeNull?.DoStuffAsync();
    }
    async Task DoStuffAsync() { /* ... */ }
}

Expected behavior

VSTHRD110 raised on all three lines in Foo method

Actual behavior

VSTHRD110 raised on first two lines in Foo method only, not on mayBeNull?.DoStuffAsync()

• Version used: 16.8.55
• Application (if applicable): Visual Studio 16.8.5

[AArnott]

As a note when this is addressed: the fix is not simply to add an await. The existing syntax would have to properly handle the null case in order to avoid a NRE:

await (mayBeNull?.DoStuffAsync() ?? Task.CompletedTask);