! on nullable argument should suppress subsequent CS8602

[drewnoakes]

Version Used: 3.3.0-beta1-19327-04

Steps to Reproduce:

bool _initialised;

void M()
{
    _initialised = true;

    object? service = GetService();

    EnsuresNotNull(service!); // ! should indicate 'service' is not null below

    service.ToString();
//  ^^^^^^^ CS8602
}

object? GetService() => _initialised ? "" : null;

void EnsuresNotNull<T>(T t) where T : class
{
    if (t is null)
        throw new ArgumentNullException();
}

It seems we don't have a way of annotating EnsuresNotNull to say that it throws if its argument is null (DoesNotReturnIf in #36810 gets close).

In the absence of that, I'd be happy to ! arguments to such guard methods. It feels consistent and local. It'll also be easy to regex out of code later if I'm able to annotate the guard function in future.

[jcouv]

The recommended way of doing this is Debug.Assert(service != null);.
The current behavior is by-design: the suppression (service!) returns a value with a non-null state, but does not change the known state of the variable service.

[RikkiGibson]

Ideally the method EnsuresNotNull could be annotated EnsuresNotNull([NotNull] object? param) to tell the compiler that "if the method returns, the argument is not-null".

[jcouv]

I'll go ahead and close this issue as by-design. Here's the relevant section of the spec. It's probably captured in the LDM notes for nullability as well, but I couldn't find which one from a quick look.