Range variables always considered not null, leading to absent CS8604 warnings

[Foxite]

Version Used: 3.4.0-beta4-19569-03 (82f2e25)

Steps to Reproduce:

class Program {
	static void Main(string[] args) {
		// The example code is a bit contrived but the problem appears in real code.
		IEnumerable<int> results =
			from arg in args
			let parsedArg = ParseString(arg) // ParseString returns `string?`. However, parsedArg is considered a `string`, not a `string?`, which you can see when inspecting the variable in VS.
			//where !(parsedArg is null) // Removal of this line should cause a CS8604 error below, but it doesn't.
			select int.Parse(parsedArg);

		string? thing = null;
		ParseString(thing); // As a sanity check, this does cause a CS8604.
	}

	private static string? ParseString(string input) {
		// Imagine this function sometimes returns null and sometimes it doesn't.
		return null;
	}
}

Expected Behavior: the parsedArg range variable is considered a string?, and removal of the where clause causes a CS8604 (Possible null reference argument for parameter) warning in the line below it.

Actual Behavior: The variable is considered a string and no warnings are emitted.

[RikkiGibson]

I think the part where parsedArg has not-null flow state is a bug, and the part where the where clause is expected to change the flow state is a feature request in the vein of dotnet/csharplang#8383 and #39586