switch expression with not explicit yield value should work ?

[Tagir Valeev]

Hello!

I think this is not related to recent JEPs. This behavior is
standardised since Java 14 when Switch expression was introduced:

// Compilation error
int x = switch(0) {
    default -> throw new IllegalArgumentException();
};

This is explicitly specified (15.28.1) [1]:

> It is a compile-time error if a switch expression has no result expressions.

There was some discussion about this rule in March, 2019 [2].
Basically, the idea is to preserve the possibility of normal
(non-abrupt) execution of every expression.
I believe, preventing unreachable code has always been in the spirit
of Java. In your code sample, the execution of the 'return' statement
itself is unreachable,
so writing 'return' is redundant. In my sample above, the 'x' variable
is never assigned to anything, and the subsequent statements (if any)
are unreachable as well.

I'd vote to keep the current behavior. While it may complicate code
generation and automatic refactorings, this additional complexity is
only marginal. The benefit is
that this behavior may save us from accidental mistakes.

Btw, you may deceive the compiler introducing a method like

static Object fail() {
  throw new IllegalArgumentException();
}

And use "case Object __ -> fail()"

With best regards,
Tagir Valeev.

[1] https://docs.oracle.com/javase/specs/jls/se16/html/jls-15.html#jls-15.28.1
[2] https://mail.openjdk.java.net/pipermail/amber-spec-experts/2019-March/001067.html

On Sun, Aug 29, 2021 at 9:00 PM Remi Forax <forax at univ-mlv.fr> wrote:
>
> Another case where the spec is weird,
> i've converted a project that generate a visitor from a grammar (something like yacc) to use a switch on type instead.
>
> Sometimes for a degenerate portion of the grammar i've an empty visitor that always throw an exception,
> the equivalent code with a switch is
>
>   static Object result(Object o) {
>     return switch (o) {
>       case Object __ -> throw new AssertionError();
>     };
>   }
>
>
> Obviously i can tweak the code generator to generate
>
>   static Object result(Object o) {
>     throw new AssertionError();
>   }
>
> but not be able to compile the former code strike me as odd.
>
> An expression switch is a poly-expression, so the result type is back-propagated from the return type of the method result, so it should be Object.
>
> Moreover, if the switch is not a switch expression but a switch statement, the code is also valid
>
>   static Object result(Object o) {
>     switch (o) {
>       case Object __ -> throw new AssertionError();
>     }
>   }
>
> Not be able to compile a switch expression when there is no explicit result type but only an implicit type seems arbitrary to me
> (this change is backward compatible because it only makes more codes compiling).
>
> Rémi