Assert.ThatAsync does not support polling

[Dreamescaper]

        var i = 0;
        async Task<string> GetResult()
        {
            return new string('1', i++);
        }

        await Assert.ThatAsync(GetResult, Is.Not.Empty.After(1000, 10));

Expected Result:
GetResult function returns non-empty result on second execution. So assertion should pass, since it has polling interval.

Actual Result:
Nunit invokes GetResult only once, therefore it fails.

Assert.That(GetResult, Is.Not.Empty.After(1000, 10))
  Expected: not <empty> after 1000 milliseconds delay
  But was:  <string.Empty>

[manfred-brands]

Looking at the code, it indeed will not and isn't likely to change unless all the underlying code changes and we introduce async versions of Constraint.Apply

At the moment ThatAsync is a small wrapper on which does:

var result = await code();
Assert.That(() => result, constraint, message, actualExpression, constraintExpression);

The existing Assert.That works with your code but will use the AsyncToSyncAdapter

Assert.That(GetResult, Is.Not.Empty.After(1000, 10));
Assert.That(i, Is.EqualTo(2));

For now we could add an issue to nunit.analyzers to warn users that ThatAsync doesn't work with DelayedConstraint.

[Dreamescaper]

unless all the underlying code changes and we introduce async versions of Constraint.Apply

That seems like a correct thing to do.
That would simplify other constraints as well, like ThrowsConstraint. Currently there is a wokraround with try-catch inside of ThatAsync.