[swift-evolution] [Pitch] Parse expressions after 'as' and 'is' instead of just types

[Matthew Johnson]

> On May 18, 2016, at 2:55 PM, Joe Groff <jgroff at apple.com> wrote:
> 
>> 
>> On May 18, 2016, at 12:54 PM, Matthew Johnson <matthew at anandabits.com> wrote:
>> 
>> 
>>> On May 16, 2016, at 4:06 PM, Joe Groff via swift-evolution <swift-evolution at swift.org> wrote:
>>> 
>>> Currently, we parse a type after 'as[?!]' and 'is'. This is mostly what you'd expect, but does lead to problems when an 'as' expression appears as part of a comparison:
>>> 
>>> 	20 as Int64 < y as Int64 // error, '>' expected to close generic parameter list Int64<y>
>>> 
>>> Looking to the future, many people have also expressed interest in the ability to do dynamic type checks against metatype values, not only static types, as in:
>>> 
>>> 	class Base {}
>>> 	class DerivedA {}
>>> 	class DerivedB {}
>>> 
>>> 	var x: Base.Type = DerivedA
>>> 
>>> 	DerivedA() as? x // succeeds
>>> 	DerivedB() as? x // fails
>> 
>> The dynamic check itself makes sense but it isn’t clear what type these expressions should return?  It can’t be `DerivedA?` like it would in the equivalent static expression as the type is not known statically:
>> 
>> DerivedA() as? DerivedA // succeeds with value Optional<DerivedA>.Some
>> DerivedB() as? DerivedA // fails with value Optional<DerivedA>.None
> 
> Right, the static type of a cast like this would still have to be the upper bound type of the metatype value. (Unless we introduce some sort of path-dependent types to allow `x.Self` to be a type.)

So `Base?` in this case?  That makes sense.  Thanks for clarifying.

> 
> -Joe

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