Switch expressions -- some revisions

[Guy Steele]

> On Dec 14, 2017, at 4:22 PM, Brian Goetz <brian.goetz at oracle.com> wrote:
> 
> . . .
> 
> So we dusted off an old idea, which we'd previously explored but which had some challenges, which is to use "break" with an operand instead of "return" to indicate local return in switch expressions.  So: 
> 
>     int y = switch(abs(x)) { 
>         case 1 -> 1; 
>         case 2 -> 2; 
>         case 3 -> 3; 
>         default -> { 
>             println("bigger than 3"); 
>             break x; 
>         } 
>     }; 
> 
> The challenge is ambiguity; this could be interpreted as a nonlocal break out of an enclosing loop whose label is `x`.  But then we realized that if `x` is both a variable and a label, we can just reject this, and tell the user to rename one or the other; since alpha-renaming the label is always source- and binary-compatible, the user has at least one (if not two) reasonable choices to get out of this problem. . . .

By all means, reject the ambiguous situation; this sounds fine.  But also advise the user of another possibility: if the intent is to return the value of the variable `x` (rather than use the label), just use parentheses and write `break (x);`.  Which I might take to doing in all cases anyway, just to make it _immediately_ clear even in nonambiguous cases that `x` is not a label.

—Guy