Metamodel: Mixins vs Extends discrepancy

[TheAngryByrd]

👋 Hey there. I'm working on a generator for F# types using the metamodel. I'm nearly there, however I've run into a what I think may be an discrepancy with the metamodel.

There are some types that are listed as extends in the LSP Spec page but are listed as mixins in the metamodel.

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For brevity the definitions of these are:

• extends: "Structures extended from. This structures form a polymorphic type hierarchy."
• mixins: "Structures to mix in. The properties of these structures are copied into this structure. Mixins don't form a polymorphic type hierarchy in LSP."

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Example of discrepancy:

ImplementationOption lists WorkDoneProgressOptions in the sepc:

export interface ImplementationOptions extends WorkDoneProgressOptions {
}

but in the metamodel it's listed as a mixin:

{
    "name": "ImplementationOptions",
    "properties": [],
    "mixins": [
        {
            "kind": "reference",
            "name": "WorkDoneProgressOptions"
        }
    ]
},

There's also a similar situation for WorkDoneProgressParams.

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I'm unsure which is the case, maybe this is my misunderstanding about typescript in general.

Any guidance would be appreciated. Thanks!