IDE0370 false positive, when initializing auto-property with `null!`.

[PetSerAl]

Version Used:
Locally: SDK 10.0.101
lab.razor.fyi: 5.0.0 and 5.3.0-2.25619.2

Steps to Reproduce:

class C
{
    public C() { }
    public string S { get; set; } = null!; //IDE0370: Suppression is unnecessary 
}

https://lab.razor.fyi/#42pi5OIKKMpPL0rM1UsuFipJzkksLlZw5uWq5uVSUFBQKChNyslMVnDW0FSoVqhFESsuKcrMS1cIVqhWSE8tsVYoBhG1CrYKeaU5OYrWCvr6ni6uBsbmBlYKwaUFBUWpxcWZ-XkKmcUKpXl5qcmpxcWJRZUKvFy1XmypRUX5RcVRnBxvfpw5-SFNgCWBsYIRAA

20260103-1611-05.8100910.mp4

IDE0370 on ! sign.

Diagnostic Id:
IDE0370: Suppression is unnecessary

Expected Behavior:
No diagnostic should be produced, as null assigned to not nullable property.

Actual Behavior:
False positive IDE0370. When proposed fix applied, it result in "CS8625: Cannot convert null literal to non-nullable reference type." warning.
No diagnostic, if constructor removed/commented.

[CyrusNajmabadi]

Something weird is happening here. We call:

            var updatedDiagnostics = updatedSemanticModel.GetDiagnostics(groupSpan, cancellationToken);

With the span of the property. And we get nothing back. But if i call GetDiagnostics on the full semantic model, then it returns that diagnostic within that span.

[CyrusNajmabadi]

Only repros if you have:

    public C() { }

[CyrusNajmabadi]

Seems to be an issue with GetDiagnostics when passed a span. Created repro and have pulled in compiler dev to see if we can figure out waht's up.

[CyrusNajmabadi]

ok. Have verified that hte compiler doesn' tproperly return warnings whne asking for diagnostics for the span for, say, a full property declaration.

It also does not return anything here even if you first ask for diagnostics for a constructor in the type. if you do ask for diagnostics for the type-declaratio span itself, these will show up. But that's not super desirable given that a type declaration may be super large itself.

ideally, from talking to @RikkiGibson , the compiler should see that the span is within a property decl and do the appropriate binding of constructors to make nullable warnings show up.

--

If, however, we can't get the GetDiagnostics side fixed, a poor-man's solution here is to update the analyzer to instead bind the entire type declaration, then filter the diagnostics to those that intersect the field/property.