Incorrect nullability analysis for extension method with inference

[jcouv]

In the example below, the only argument for the extension method is oNull which may be null.
So oNull.Id has signature object? ().

The conversion to an explicit type works properly (warn on nullability issue with return type).
But in the function type scenario, it looks like var x isn't analyzed with that inferred type.

Also, the semantic model (GetSymbolInfo) has incorrect nullability for both member accesses.

This issue also impacts the semantic model for deconstructions.

This issue also impacts new extension members.

#nullable enable

object? oNull = null;
var x = oNull.Id;
x().ToString(); // Missing nullabiliy warning, indicating that the type inferred for `var` is incorrect

System.Func<object> y = oNull.Id; // correct warning on return type of delegate type

static class E
{
    public static T Id<T>(this T t) => t;
}