[swift-dev] [SE-0143] Dynamic casting for conditional conformances

[Jordan Rose]

> On Dec 12, 2017, at 14:15, Douglas Gregor via swift-dev <swift-dev at swift.org> wrote:
> 
> 
> 
>> On Dec 10, 2017, at 4:47 PM, Brent Royal-Gordon <brent at architechies.com <mailto:brent at architechies.com>> wrote:
>> 
>>> On Dec 5, 2017, at 2:28 PM, Douglas Gregor via swift-dev <swift-dev at swift.org <mailto:swift-dev at swift.org>> wrote:
>>> 
>>> To perform that mapping from a mangled type in a conditional requirement to type metadata, we effectively need an operation to take a mangled type name and turn it into a type metadata pointer. This is something we could surface in the Swift standard library/runtime as, e.g.,
>>> 
>>> func type(named: String) -> Any.Type?
>>> 
>>> to take a mangled type name and try to get the type metadata for it. From there, one can query protocol conformances that (say) allow one to construct instances of the arbitrarily-named type. Think of it as NSClassFromString for any type in the Swift language, including specializations of generic types.
>> 
>> 
>> It's worth noting here that the standard library already provides a limited `_typeByName(_:)` function for Corelibs Foundation to use. That function will presumably become part of the ABI unless we design a public version in Swift 5.
> 
> 
> Woah. That’s kinda awful… it parses “Foo.Bar” as a class name and looks for class Bar in module Foo.

Needed to implement NSCoding on Linux. They did limit it to "Foo.Bar" at least, deliberately excluding anything more complicated.

Jordan

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