It appears that you can’t extend a generic class/struct with a requirement that
a type parameter inherit from a non-protocol:
extension Dictionary where Key : String { … }
The above produces the error “Type ‘Key’ constrained to non-protocol type
‘String’”.
So is there a way I can add a method to a class, where the method depends on
one of the type parameters being a specific type? For example, let’s say I want
a “concatenatedKeys” method that returns all the Dictionary’s keys concatenated
into a string with commas between them.
(The actual reason I’m doing this is more complicated, but it has to do with
JSON encoding — JSON dictionary keys have to be strings, so I want to be
type-safe and make my encoding methods available only for Dictionary<String,
___>.)
—Jens
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