SE-0267 — `where` clauses on contextually generic declarations

[anthonylatsis]

This is what Slava originally said on this matter:

«where» clauses on contextually generic declarations

It would be possible but I don't think we want to do that. Mangling for nested entities intentionally reflects the lexical structure, so that, for example, Demangle::getTypeForMangling() can easily find the declaration. If you mangle declarations with a where clause as if they were part of a fake constrained extension it would complicate code like that. Also, we already mangle generic declarations with where clauses that reference outer parameters differently from constrained extension members…

Though I don't see how the lexical structure breaks when we exchange constraints between the extension and the member.

@jrose

* We can't canonicalize to the bottom-right cell because it's not obvious which constraints go on the extension and which go on the function.

Does it matter at all to the demangler where the constraints are placed?

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@anandabits

I understand why a protocol requirement isn't allowed to be conditional by constraining Self or associated types. Can you clarify whether declarations in protocol extensions will be allowed to use constraints based on Self or associated types? It seems like this should be allowed and would be very useful.

Yes, that will be allowed too since you can already constrain the extension instead.