[Webkit-unassigned] [Bug 239668] New: [Wasm-GC] Add support for subtyping declarations

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=239668

            Bug ID: 239668
           Summary: [Wasm-GC] Add support for subtyping declarations
           Product: WebKit
           Version: WebKit Nightly Build
          Hardware: Unspecified
                OS: Unspecified
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: WebAssembly
          Assignee: webkit-unassigned at lists.webkit.org
          Reporter: asumu at igalia.com

As mentioned in https://bugs.webkit.org/show_bug.cgi?id=239666, full support of the Wasm GC proposal requires type system support for both recursion groups and subtyping declarations.

In the latest GC proposal design, subtyping (e.g., for struct types) is declared explicitly with a `sub` operator:

Example:


```
(module
  (type $s1 (struct))
  (type $s2 (sub $s1 (field i32)))
)
```

The binary format allows for multiple supertypes, but the MVP design will restrict it to one supertype.

A reference type with an index to a type definition that has supertypes will be compatible with a reference type to its supertype index.

Only array and struct types will support subtyping at first, so it likely makes sense to implement this after struct operations and recursive types land.

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