Yield as contextual keyword

[Guy Steele]

> On May 24, 2019, at 10:34 AM, Brian Goetz <brian.goetz at oracle.com> wrote:
> 
> 
> var yield = 5;
> 
> yield is lexed as an identifier, so this is a valid variable declaration
> 
>  var res = switch(yield) {
> 
> yield is lexed as an identifier, so this is a valid switch operand
> 
> 
>     default -> yield + yield;
> 
> RHS of a single-consequence case is expression | block statement | throw.  We're not in a YieldStatement production, so this is an expression.
> 
> Like lambdas, we can think of this as shorthand for
> 
>     default -> {
>         yield yield + yield;
>     }
> 
> which parses as a yield statement whose operand is the expression yield+yield.

And, yay (thanks, Tagir),

    default -> {
        yield + yield;
    }

is a genuine, bona fide puzzler!

(But as long as we are doing special-case parsing and checking the next token right after an occurrence of yield, it is a puzzler we can easily defend against if we want to.)

—Guy