[Nullable ref Types] Infer throw statements from extension methods

[garuma]

Version Used: VisualStudio.16.int.d16.0stg/16.0.0-pre.2.0+28408.101.d16.0stg
C# Tools 2.11.0-beta2-63529-05+66aa49da81717aa54090a9e94c2f575ddb1959bb

Steps to Reproduce:

Simplified definition:

struct TwoEnumeratorListStack
{
    private readonly ArrayBuilder<Which>? _discriminatorStack;

    public Which PeekNext()
    {
        // Possible dereference of a null reference
        return _discriminatorStack[_discriminatorStack.Count - 1];
    }
}

To prevent the warning you can do:

public Which PeekNext()
{
    if (_discriminatorStack == null)
        throw new InvalidOperationException();
    return _discriminatorStack[_discriminatorStack.Count - 1];
}

Which removes the warning. However trying to capture the check in a general-purpose extension method:

internal static void ThrowInvalidIfNull<T>(this T? argument) where T : class
{
    if (argument == null)
        throw new InvalidOperationException();
}

public Which PeekNext()
{
    _discriminatorStack.ThrowInvalidIfNull();
    return _discriminatorStack[_discriminatorStack.Count - 1];
}

Doesn't remove the warning anymore and requires additional ! to both array indexer and .Count

[jaredpar]

Thanks for reporting! This scenario, and others, are covered in the following issue. The intent is definitely to support this

#32078

[jcouv]

The [System.Runtime.CompilerServices.AssertsNotNull] attribute can be applied on the parameter of ThrowInvalidIfNull to help the compiler draw the correct inference.
You may have to define the attribute yourself for now, but we plan to add it to Core 3.