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

[Joe Groff]

> On May 16, 2016, at 5:49 PM, Dan Appel <dan.appel00 at gmail.com> wrote:
> 
> Just to clarify: in your example, did you mean for DerivedA and DerivedB to inherit from Base?

Yeah, sorry about that.

-Joe

> 
> On Mon, May 16, 2016 at 4:18 PM Matthew Johnson via swift-evolution <swift-evolution at swift.org> wrote:
> +1
> 
> Sent from my iPhone
> 
> > 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
> >
> > If we accept https://github.com/apple/swift-evolution/blob/master/proposals/0090-remove-dot-self.md, dropping the '.self' requirement to refer to type objects, then I think we should also change 'is' and 'as' to parse the expression grammar on their right-hand side, leaving it up to the normal expression disambiguation rule to handle angle brackets. This solves the '20 as Int64 < x' problem, and prepares us to support dynamic is/as queries in the future. (To be clear, designing dynamic queries should be its own discussion.) What do you all think?
> >
> > -Joe
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> Dan Appel