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

[Joe Groff]

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