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

[Tony Allevato]

+1

On Mon, May 16, 2016 at 2:07 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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