Local of unconstrained type parameter type is reported as NotAnnotated when using var

[cston]

The local returned from SemanticModel.GetDeclaredSymbol() for y1 has Type.NullableAnnotation == NullableAnnotation.NotAnnotated, but should be NullableAnnotation.Annotated instead.

Compare with the GetDeclaredSymbol() result for y2 which has Type.NullableAnnotation == NullableAnnotation.Annotated.

#nullable enable

class Program
{
    static T F1<T>(T x1)
    {
        var y1 = x1;
        y1 = default(T);
        return y1;
    }

    static T F2<T>(T x2) where T : class
    {
        var y2 = x2;
        y2 = default(T);
        return y2;
    }
}

[RikkiGibson]

@333fred is this by design, since we tend to "lie" about the nullable annotation to give a consistent answer to what we give for flow state? e.g. we always give (NotAnnotated, NotNull) or (Annotated, MaybeNull) or "oblivious"?

follow-up: the "lie" only occurs for API like GetTypeInfo, which is intended to give you an rvalue type. The API in this issue description is GetDeclaredSymbol which is expected to provide the real annotation for the symbol.

[333fred]

It might have been by design in C# 8, but with T? in C# 9 it no longer feels like that's the case.

[RikkiGibson]

The initial state of parameters, though, is MaybeNull for both unconstrained T and T?, and that is where the NullableAnnotation.Annotated is coming from in this public API, right?

[333fred]

I don't believe we're talking about parameters here. We're talking about var. And var should be annotated, with whatever flow state it gets from the initializer.

[RikkiGibson]

Follow-up to offline discussion: this API is supposed to tell the truth 😉 so this is a bug. Thanks for helping me understand the scenario.

I think that even if the API were supposed to give an annotation reflecting the "rvalue type", this would still be the wrong behavior--I must have just gotten mixed up when reviewing the description.

[jcouv]

Fixed and verified for 17.0 (p1 or p2) by #53691

[jcouv]

Also backported fix to 16.11