DiagnosticsSuppress does not suppress CS8634

[manfred-brands]

Given the following constructed example:

[Test]
public void Test()
{
    object? possibleNull = GetNext();
    Assert.NotNull(possibleNull);
    DoNothing((object)possibleNull);
}

private static object? GetNext() => new object();

private void DoNothing<T>(T obj)
    where T : class
{}

Result in error:

CS8634 The type 'object?' cannot be used as type parameter 'T' in the generic type or method 'TestClass.DoNothing<T>(T)'. Nullability of type argument 'object?' doesn't match 'class' constraint.

Even trying to be explicit, doesn't remove the error:

    object? possibleNull = GetNext();
    Assert.NotNull(possibleNull);
    object notNull = (object)possibleNull;
    DoNothing(notNull);

This might be a limitation on the DiagnosticSuppressors where we suppress the error generated for the cast, but we don't change the compiler's understanding of what could be null. We don't do a complete flow analysis on notNull

The only option that seems to work is by either using the null suppression operator: object notNull = possibleNull!;
or by doing the nullability conversion in a separate method:

[Test]
public void Test()
{
    object? possibleNull = GetNext();
    object notNull = EnsureNotNull(possibleNull);
    DoNothing(notNull);
}

private static object? GetNext() => new object();

private static object EnsureNotNull(object? possibleNull)
{
    Assert.NotNull(possibleNull);
    return (object)possibleNull;
}

private void DoNothing<T>(T obj)
    where T : class
{}