False positive 'possible null reference argument' after ensuring non-null via the null coalescing operator

[Treit]

Repro

The following program results in a warning about passing a possible null argument to DoSomething. As far as I can see there is no possibility the argument could be null.

using System;
using System.Collections.Generic;

var ids = GetIds();

if ((ids?.Count ?? 0) == 0)
{
    return;
}

DoSomething(ids);

static void DoSomething(List<string> ids)
{
    Console.WriteLine(ids.Count);
}

static List<string>? GetIds()
{
    return null;
}

Replacing if ((ids?.Count ?? 0) == 0) with if (ids is null || ids.Count == 0) (or various other pattern matching techniques to accomplish the same thing) makes the warning go away, even though Sharplab says they lower to the same code.

We have this pattern in many places in a large codebase and it makes turning on nullable checks difficult. It would be much nicer if the compiler could detect that the variable cannot be null in this case.

[CyrusNajmabadi]

Closing out. The compiler doesn't track the null relationship through arithmetic expressions (like (ids?.Count ?? 0) and then teh ==0).

You can write this as:

if (ids is null || ids.Count == 0)
{
    return;
}

or

if (ids is not { Count: > 0 })
    return;

It would be much nicer if the compiler could detect that the variable cannot be null in this case.

We've looked at this sort of relationship before and decided the large increase in analysis complexity is not worth it, given hte simple and idiomatic transforms available.