[code-reflection] RFR: Unreachable method or lambda exit [v2]

[Paul Sandoz]

On Fri, 18 Oct 2024 09:20:11 GMT, Maurizio Cimadamore <mcimadamore at openjdk.org> wrote:

> Looks good. I wonder if something similar is required for switch/yield?
> 
> E.g.
> 
> ```
> jshell> String s = switch (n) {
>    ...>     case 3 -> { while (true);}
>    ...>     default -> " Hello!";
>    ...> };
> s ==> " Hello!"
> ```

Drat, i did briefly ponder that but did not realize such switch expressions would be legal code.  Is this a bug or some sort of compromise?

This is contrary to what [JEP 361](https://openjdk.org/jeps/361) says:

> Furthermore, a switch expression must either complete normally with a value, or complete abruptly by throwing an exception. This has a number of consequences. First, the compiler checks that for every switch label, if it is matched then a value can be yielded.

This is also contrary to what the JLS states in section [15.8](https://docs.oracle.com/javase/specs/jls/se23/html/jls-15.html#jls-15.28):

> A switch expression transfers control to one of several statements or expressions, depending on the value of an expression; all possible values of that expression must be handled, and all of the several statements and expressions must produce a value for the result of the switch expression. 

But digging deeper i think it comes down to this in section [15.18.1](https://docs.oracle.com/javase/specs/jls/se23/html/jls-15.html#jls-15.28):

> It is a compile-time error if the switch block of a switch expression consists of switch rules, but one or more switch rule blocks can complete normally ([§14.22](https://docs.oracle.com/javase/specs/jls/se23/html/jls-14.html#jls-14.22)).

There is a tension here. In the example the switch expression cannot complete normally with a value, or abruptly with an exception, because the while statement does not complete normally and therefore the enclosing switch rule block does not complete normally, but it also does not complete abruptly.

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PR Comment: https://git.openjdk.org/babylon/pull/257#issuecomment-2423218614