`on target delete delete source` does not delete object extending abstract type

[dermetfan]

• EdgeDB Version: 1.0-alpha.7+g85a3931a7.d20210113 from Docker Hub
• OS Version: Official Docker image running on NixOS 21.03pre-git (x86_64)

Steps to Reproduce:

1. INSERT B { value := (INSERT C) };
2. DELETE C;

Now B should be deleted through the A.value link target deletion policy, but it was instead set empty. Check with SELECT B { value };.

Schema:

abstract type A {
	link value -> C {
		on target delete delete source;
	};
}

type B extending A {}

type C {}

[elprans]

@msullivan, this looks like the issue you've been looking into recently.