Constraint Throws.Exception does not work with async return value

[origameg]

When using the Throws.Exception constraint to test an async delegate with a return value, the test incorrectly fails even when an Exception should have been thrown:

[Test]
public void ThrowsExceptionConstraint_AsyncReturnValue_Fails()
{
    Assert.That(async () => { await Task.Delay(200); throw new Exception(); return true; },
        Throws.Exception);
}

Failed   ThrowsExceptionConstraint_AsyncReturnValue_Fails
Error Message:
   Expected: an exception to be thrown
  But was:  no exception thrown

The constraint appears to work fine for async delegates that return Task instead of Task<T> or if additional constraints are supplied.

[Test]
public void ThrowsExceptionConstraint_AsyncTask_Passes()
{
    Assert.That(async () => { await Task.Delay(200); throw new Exception(); },
        Throws.Exception);
    // Passes
}

[Test]
public void ThrowsExceptionConstraint_AsyncReturnValue_AdditionalConstraint_Passes()
{
    Assert.That(async () => { await Task.Delay(200); throw new Exception("foo"); },
        Throws.Exception.Message.Contains("foo"));
    // Passes
}

I'm aware that it is not recommended to use the Throws.Exception constraint on its own. However, it's not completely uncommon, and the false test failure is pretty misleading.

I've reproduced this on:

• NUnit versions: 3.11.0, 3.10.1
• Test runners: nunit3-console v3.9.0, ReSharper 2018.1.2
• .NET versions: Core 2.0, Core 2.1, Framework 4.6.1

[mikkelbu]

We should probably generalise the logic (see https://github.com/nunit/nunit/pull/2547/files#diff-528b529b1298bbb4bd387f02090a3de3R105) from #2547 to also handle Task<T>.

[stevenaw]

Thanks for the pointers @mikkelbu .
I can take this on.