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Add a feature specification for the metaobject proposal #4301

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@eernstg eernstg commented Mar 24, 2025

This PR adds a feature specification for the metaobject proposal (formerly known as 'more capable Type objects'). For simplicity, it does not include the part about Type.reify (that will be specified separately).

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Lots of improvements, PTAL!

The change to a more abstract approach does get a little bit silly when we have to reinvent covariant along the way, but it does also make some other things simpler. I think it's a slight improvement on average.

However, it's a major point in the design of this feature that it does not introduce a lot of completely new from-scratch primitive semantics, it is simply a way to transform a type C<T1 .. Tk> to an object MetaC<T1 .. Tk>, and all the expressive power follows from that, based on well-established mechanisms like superinterfaces, overriding declarations, etcetc.

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