I spoke with Simon today, and I think I have a bit of a better idea now of what's going on with specialization, and why it sometimes fails to specialize things as much as it could. Apparently, the replacement of (sel @ type dict) by sel.type is accomplished by the use of a rewrite rule generated by the specializer when it specializes an overloaded function using that (or something similar to that, anyway). The trouble is that as things are currently done, there's no way in general to get the right replacement to stick on the RHS of the rule when such forms arise in other contexts. The type checker constructs dictionaries in type checking, but afterwards we can't produce new ones.
The most likely way to solve this is probably to (somehow) decouple dictionary creation and selection from type checking so as to expose those facilities to later phases. The concept would be something vaguely like this: instead of creating rewrite rules, the specializer would instead check whether the appropriate dictionary already existed, and create it if not. The simplifier would then check for a dictionary every time it encountered a class method whose instance is determined. No, I have no idea how much work such a change would involve.
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