language reference entry for non-extending subtypes

[ginsbach]

fixes https://github.com/github/codeql-coreql-team/issues/1494

[smowton]

Is this functionally identical to extending Foo's base types and writing this instanceof Foo in Bar's charpred? If so, that might be useful to mention.

[ginsbach]

Is this functionally identical to extending Foo's base types and writing this instanceof Foo in Bar's charpred? If so, that might be useful to mention.

It isn't quite the same. To be specific, it goes beyond just adding it to the charpred in two ways:

• the relationship is recorded in the type hierarchy, making type-based optimisations stronger
• it is possible to access member predicates through the super keyword

[alexet]

I think being clearer about which things care about extends and which are instanceof earlier in this document may help.

I think the comparison to instanceof would actually be good saying something like
"Foo instanceof Baris reoughly equivalent to sayingthis instanceof Barin the charpred. The main differences are that you can call methods onBarviasuper` and you can get better optimisation.

[alexet]

Again I think the overlapping instanceof/extends hierarchy is just confusing here. I would have a simpler example like the following:

    class Interface instanceof int {
      Interface() { this in [1 .. 100] }
      string foo() { result = "" }
   }

    class Foo extends int {
      Foo() { this in [1 .. 10] }
      string foo() { result = "foo" }
    }

    class Bar extends Interface instanceof Foo {
      override string foo() { result = "bar" }
    }

Here we don't need to understand the subtle differences between rootdefs and where exactly overriding happens.

Also your current example seems wrong select any(Foo b).foo() should call Interface::foo which should include "bar" as Bar::foo overrides Interface::foo. This seems an implementation bug.

[ginsbach]

I have opened an issue (https://github.com/github/codeql-coreql-team/issues/1513) for the implementation bug.

I attempted to take your example, but the toString methods became messy. Does the following make sense to you (commit 9)?

    class Interface extends int {
      Interface() { this in [1 .. 10] }
      string foo() { result = "" }
   }

    class Foo extends int {
      Foo() { this in [1 .. 5] }
      string foo() { result = "foo" }
    }

    class Bar extends Interface instanceof Foo {
      override string foo() { result = "bar" }
    }

[ginsbach]

The implementation bug should be fixed now: https://github.com/github/semmle-code/pull/40188

[ginsbach]

Thank you for the detailed suggestions @alexet !

Foo instanceof Baris reoughly equivalent to sayingthis instanceof Barin the charpred. The main differences are that you can call methods onBarviasuper` and you can get better optimisation.

I have added a sentence like this in commit 8.