Type pattern: definite assignment false positive for struct that has only reference type properties and is declared in another assembly

[HellBrick]

Version Used:
Visual Studio 16.2.5
MSBuild 16.2.37902+b5aaefc9f
csc.exe 3.2.1-beta4-19408-03

Steps to Reproduce:
You need two assemblies for this, Lib and App, where App references Lib.

// Lib
namespace Lib
{
	public struct StructWithStringProp
	{
		public string Text { get; set; }
	}
}

// App
namespace App
{
	internal class CallSites
	{
		private static string ExternalStringPropCallSite(Lib.StructWithStringProp? nullable)
		{
			bool flag = nullable is Lib.StructWithStringProp notNull;
			return flag ? notNull.ToString() : "";
		}
	}
}

Expected Behavior:
error CS0165: Use of unassigned local variable 'notNull'

Actual Behavior:
The code compiles.

Additional information:
If you make any of the following changes, the code stops compiling and the expected error appears:

• replace struct's property with a field
• change struct's property type from string to, let's say, int
• move struct declaration from Lib to App

You can check out a solution that illustrates all these permutations and their effect on the compiler behavior here.

[jcouv]

I suspect this is a known issue due to the struct being empty. That said, I'm not sure why this would only affect an empty struct defined in metadata (as opposed to the same compilation/source).
Tagging @gafter to confirm.

[gafter]

This is an intentional behavior. See also #30194 where we propose to fix this intentionally broken behavior. The struct contains only a private (in this case compiler-generated) field of a reference type, so (due to the intentional behavior of the compiler) it is ignored for the purposes of definite assignment when imported from an assembly.